Respiratory Syncytial Virus Bronchiolitis in Infancy Is an Important Risk Factor for Asthma and Allergy at Age 7

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1 Respiratory Syncytial Virus Bronchiolitis in Infancy Is an Important Risk Factor for Asthma and Allergy at Age 7 NELE SIGURS, RAGNAR BJARNASON, FRIDRIK SIGURBERGSSON, and BENGT KJELLMAN Department of Pediatrics, Borås Central Hospital, Borås, Sweden; Department of Pediatrics, Sahlgrenska University Hospital, Gothenburg, Sweden; Emergency Department, Reykjavik Hospital, Reykjavik, Iceland; and Department of Pediatrics, Skövde Central Hospital, Skövde, Sweden We previously reported an increased risk for bronchial obstructive disease and allergic sensitization up to age 3 in 47 children hospitalized with a respiratory syncytial virus (RSV) bronchiolitis in infancy compared with 93 matched control subjects recruited during infancy. The aims of the present study were to evaluate the occurrences of bronchial obstructive disease and allergic sensitization in these children at age 7 1 / 2. All 140 children reported for the follow-up, which included physical examination, skin prick tests, and serum IgE tests for common food and inhaled allergens. The cumulative prevalence of asthma was 30% in the RSV group and 3% in the control group (p 0.001), and the cumulative prevalence of any wheezing was 68% and 34%, respectively (p 0.001). Asthma during the year prior to follow-up was seen in 23% of the RSV children and 2% in the control subjects (p 0.001). Allergic sensitization was found in 41% of the RSV children and 22% of the control subjects (p 0.039). Multivariate evaluation of possible risk factors for asthma and sensitization using a stepwise logistic statistical procedure for all 140 children showed that RSV bronchiolitis had the highest independent risk ratio for asthma (OR: 12.7, 95% CI 3.4 to 47.1) and a significantly elevated independent risk ratio for allergic sensitization (OR: 2.4, 95% CI 1.1 to 5.5). In conclusion, RSV bronchiolitis in infancy severe enough to cause hospitalization was highly associatied with the development of asthma and allergic sensitization up to age 7 1 / 2. The results support the theory that the RSV influences the mechanisms involved in the development of asthma and allergy in children. Severe bronchiolitis in infancy is often caused by respiratory syncytial virus (RSV) (1). It has been well documented in both retrospective and prospective studies that infants with RSV bronchiolitis often develop recurrent wheezing and asthma during childhood (2 7). The mechanisms behind this development are not known. It has been a matter of speculation whether RSV actually causes asthma or just picks out those children with a predisposition for bronchial obstructive disease (8 12). Furthermore, it is not known whether or not RSV bronchiolitis merely accelerates the onset of asthma or if the differences in asthma rates seen between children with and without RSV bronchiolitis in infancy will remain in later childhood (10). Studies on the relation between RSV bronchiolitis in infancy and subsequent development of clinical allergy or allergic sensitization have shown different results: some investigators have found a connection (5, 7) whereas others have not (2 4). (Received in original form June 15, 1999 and in revised form September 28, 1999) The study was supported by grants from the Regional Health Care Authority of West Sweden, the Swedish Foundation for Health Care Sciences and Allergy Research, Pharmacia & Upjohn Diagnostics AB, Glaxo Wellcome AB, 3M Pharma, and BeDe s fund. Correspondence and requests for reprints should be addressed to Dr. Nele Sigurs, Borås Central Hospital, S , Borås, Sweden. nele.sigurs@swipnet.se Am J Respir Crit Care Med Vol 161. pp , 2000 Internet address: We compared prospectively a group of 47 children hospitalized with RSV bronchiolitis in infancy and a matched control group of 93 children at the mean ages of 1 yr and 3 yr (7). Both the frequency of asthma and of allergic sensitization was significantly higher in the RSV group than in the control group. The present study reports the results of a third followup of this cohort, at age 7 1 / 2. We aimed, first, to study the development and course of bronchial disease, clinical allergies, and sensitization and, second, to determine whether any occurrences since the second follow-up affected the comparability of the study groups. METHODS Study Groups Data on clinical symptoms as well as family history and environmental factors were obtained for all 140 children participating in the previous two follow-ups. Structured questionnaires were used. The present follow-up was done during March April (136 subjects) and May July, 1997 (four subjects). Children with RSV bronchiolitis. From December 1989 to April 1990, 52 infants (25 boys) without other concomitant chronic disease were hospitalized with RSV bronchiolitis at the Pediatric Department, Borås Central Hospital, Sweden. The diagnosis was verified using the Abbott Testpack RSV (Abbott Laboratories, Abbott Park, IL), a rapid enzyme immunoassay for direct detection of viral antigen in nasopharyngeal secretion (13). The families of 47 infants (mean age 116 d, range 30 to 307 d, 43 age 6 mo, three age 7 to 8 mo, and one child 10 mo, 21 boys) consented to their enrollment in the follow-ups and all the children reported for the first two follow-up investigations at the mean ages of 1 and 3 yr (7). At the present follow-up, 45 of the 47 children were examined and interviewed by one of the same doctors as previously (N.S., R.B., F.S.). An additional child was interviewed and examined by an experienced pediatric allergologist using the same procedure and questionnaire. The parents of the remaining child were interviewed by telephone and by means of the questionnaire delivered and returned by letter. The mean age of the 47 RSV children at the time of the investigation was 7.50 (range, 7.12 to 8.08) yr. Control group. For each infant with RSV bronchiolitis, we contacted the parents of the two infants with birth dates closest to this index infant and of the same sex and from the same local Child Health Center. Each such center covers a defined geographic area, with 99% attendance of the infants. The families were asked to participate in a follow-up study concerning the development of asthma and allergy. Three families refused to participate. The families with infants with the closest birth dates after these three not participating children were contacted and two consented. Thus, the control group consisted of 93 children (42 boys). All 93 of these children participated in the previous follow-up investigations at ages 1 and 3 yr. At the present followup, 89 children were interviewed and examined by one of the same pediatricians as previously, and four were interviewed by telephone and by means of questionnaires delivered and returned by letter. The mean age of the control subjects was 7.51 yr (range, 7.08 to 7.96 yr). Comparability Our previous analyses of the comparability of the RSV and the control groups showed that they were similar regarding 14 of 16 background factors, including family history of atopy/asthma, smoking in the family, presence of indoor furred animals, and history of pertussis

2 1502 AMERICAN JOURNAL OF RESPIRATORY AND CRITICAL CARE MEDICINE VOL and atopic dermatitis (7). The only diverging factors were a slightly shorter duration of exclusive breast feeding in the RSV group (probably due to the hospitalization) and a slightly shorter mean gestational age in that group (38.9 wk, SD 2.0 wk) compared with the control group (39.8 wk, SD 1.6 wk). Skin Prick Tests (SPT) SPTs were performed for the following allergens: egg white, peanut, hazelnut, danders (dog, horse, cat), pollens (birch and timothy), Dermatophagoides pteronyssinus, and Cladosporium herbarum. The extracts used (Soluprick) were from ALK, Hörsholm, Denmark. The test extract concentrations were 1:100 wt/vol for the food allergens and 10 histamine equivalent points (HEP) corresponding to 10,000 biological units for the inhalant allergens. Histamine solution (10 g/l) and 0.9% saline solution with human serum albumin were used for reference. The SPT was regarded as positive when the mean diameter of the wheal (half the sum of the largest diameter and its perpendicular measurement) was 3 mm or more. SPTs were performed in 44 RSV children and 89 control subjects. Serum IgE Antibodies Two screening tests for IgE antibodies were used, MultiCAP Fx5 and Phadiatop, Pharmacia CAP system (Pharmacia Upjohn Diagnostics AB, Uppsala, Sweden). The tests were performed by the Pharmacia Upjohn Diagnostics laboratory in Uppsala according to their usual protocol (14). The MultiCAP Fx5 disc checks for antibodies to egg white, soy protein, cow s milk, wheat, peanut, and fish allergens (14). The Phadiatop tests antibodies to inhalant allergens such as furred animal danders, pollens, mites, and molds (14). Sera from the children with a positive Phadiatop were investigated at the same laboratory for IgE antibodies to cat, birch, timothy, and D. pteronyssinus, using the Pharmacia CAP system (15). Values of 0.35 ku/l or greater were regarded as positive in all the serological tests. Serum tests were done in 44 RSV children and 87 control subjects. Clinical Definitions To ensure uniform classification, the records were evaluated repeatedly, but not in a blinded manner, by the investigators involved in the clinical assessment. The clinical definitions have been described in detail previously (7) and are briefly summarized here. Asthma was defined as at least three episodes of bronchial obstruction verified by a physician. Bronchial obstruction occurring at least three times but not verified by a physician was classified as recurrent wheezing. The term any wheezing was used to imply asthma, recurrent wheezing, or one or two episodes of wheezing. Allergic rhinoconjunctivitis (ARC) was defined as rhinitis/conjunctivitis appearing at least twice after exposure to a particular allergen and unrelated to infection. Atopic dermatitis (AD) was defined as a pruritic, chronic, or chronically relapsing noninfectious dermatitis, based on the suggestions of Hanifin and Rajka (16). Allergic sensitization implied occurrence of IgE antibodies estimated by SPT and/or serum IgE tests. Atopic disease in parents or siblings signifies that they had been diagnosed at some time by a physician with any of the following: asthma, ARC, or AD. Single (double) heredity for asthma or atopic disease in the 140 participating children signifies disease in one (two or more) parents and/or siblings. Statistics For comparisons between groups Fisher exact test (two-tailed) was used. Changes of proportions over time were analyzed by McNemar s test. A value of p 0.05 was regarded as significant. Relative risks (RR) and 95% confidence intervals (CI) were also calculated. To determine which hereditary and environmental risk factors were important for the development of bronchial obstructive symptoms and sensitization in the whole group of 140 children, Fisher exact test between each risk factor and each variable was first performed. To estimate which risk factors were independently related to the bronchial symptoms and sensitization, the risk factors with p 0.10 in this test were included in multivariate forward stepwise logistic regression analyses using the SAS logistic procedure. Odds ratios and 95% CI were also calculated for significant risk factors in this model. Ethics All parents gave their oral consent after receiving oral and written information. The study was approved by the Human Research Committee of the medical faculty of Gothenburg University. RESULTS A comparison of 12 background factors did not show any significant differences between the RSV group and the control group (Table 1). Cumulative and current prevalences of asthma, recurrent wheezing, any wheezing, and ARC were significantly higher in the RSV group, whereas the prevalences of AD did not differ between the groups (Table 2). Current asthma with allergic sensitization was also significantly higher in the RSV group (Table 2). Three of the 11 children with asthma at age 3 had become asymptomatic at age 7 1 / 2, as had the one child in the control group (not significant [NS]). One of the 11 children with current asthma at the present follow-up had presented with symptoms after age 3, and the other two had obstructive symptoms at age 3. One of the two control subjects with current asthma was obstructive at age 3 and the other one manifested symptoms after that age. Seven of the 19 RSV children with any wheezing at age 3 had recovered at age 7 1 / 2, as had seven of the eight control subjects (p 0.033, RR 0.42, 95% CI 0.22 to 0.88). Thus, of the 18 RSV children with any wheezing at age 7 1 / 2, six had presented with symptoms after age 3, compared with one of the two control subjects (NS). The course from 1 to 7 1 / 2 yr of age of the current asthma and any wheezing is shown in Figure 1a. The increase in current asthma from age 1 to 7 1 / 2 was not significant for the RSV group (p 0.113). The decrease in current any wheezing from age 1 to 7 1 / 2 in the control group was significant (p 0.004). Seven of the 11 RSV children and none of the two control subjects with current asthma used inhaled steroids continuously. Another two RSV children used inhaled steroids intermittently. The two children with current asthma in the control group were medicated with 2 stimulants on demand. The results of the tests used for estimating allergic sensitization are given in Table 3. Allergic sensitization was significantly higher in the RSV group, both totally and for inhalant antibodies. The course of current sensitization to inhaled allergens from age 3 to 7 1 / 2 is shown in Figure 1b. The increase in the rate of a positive Phadiatop in the RSV children was not significant (p 0.5), but a significant increase was seen among the control chil- TABLE 1 EVALUATION OF 12 BACKGROUND FACTORS IN THE 47 CHILDREN WITH RSV BRONCHIOLITIS IN INFANCY AND THEIR 93 CONTROLS AT AGE 7 1 / 2 * Background Factor RSV Group (n 47) Control Group (n 93) p Value Heredity for atopy 33 (70%) 60 (65%) 0.57 Heredity for asthma 21 (45%) 27 (29%) 0.09 Heredity for atopy, parents 29 (62%) 45 (48%) 0.15 Heredity for asthma, parents 16 (34%) 19 (20%) 0.10 Male sex 21 (45%) 42 (45%) 1.0 Smoking in family, current 20 (43%) 36 (39%) 0.72 Smoking in family cumulative 24 (51%) 49 (53%) 0.86 Indoor furred animals, current 21 (45%) 41 (44%) 1.0 Indoor furred animals, cumulative 22 (47%) 52 (56%) 0.37 No. of siblings Mean weight at follow-up, kg Mean height at follow-up, cm * Data are presented as n (%), or mean SD. No significant differences were found.

3 Sigurs, Bjarnason, Sigurbergsson, et al.: RSV and Development of Asthma and Allergy 1503 TABLE 2 NUMBER (PERCENTAGE) OF CHILDREN WITH BRONCHIAL OBSTRUCTIVE SYMPTOMS, ARC, AND AD IN THE RESPIRATORY SYNCYTIAL VIRUS GROUP AND THE CONTROL GROUP AT AGE 7 1 / 2 Symptoms* RSV Group (n 47) Control Group (n 93) p Value RR 95% CI Asthma Cumulative 14/47 (30%) 3/93 (3%) Current 11/47 (23%) 2/93 (2%) Current, atopic 7/47 1/ Recurrent wheezing Cumulative 13/47 (28%) 10/93 (11%) Current 6/47 (13%) 0/ Not calculable Any wheezing Cumulative 32/47 (68%) 32/93 (34%) Current 18/47 (38%) 2/93 (2%) ARC Cumulative 7/47 (14.9%) 2/93 (2%) Current 7/47 (14.9%) 2/93 (2%) AD Cumulative 13/47 (28%) 21/93 (23%) Current 8/47 (17%) 21/93 (13%) Definition of abbreviations: AD atopic dermatitis; ARC allergic rhinoconjunctivitis; CI confidence intervals; RR relative risk. * Current symptoms means disease during the preceding year. Current, atopic asthma means asthma with allergic sensitization. dren (p 0.027). The children with a positive Phadiatop at age 7 1 / 2 were further tested for serum IgE antibodies to cat dander, birch and timothy pollens, and D. pteronyssinus and the rate of sensitization was significantly higher in the RSV group, 29 positive tests of 56 performed, compared with 14 of 48 in the control group (p 0.028, RR 1.776, 95% CI to 2.951). The influence of a family history of asthma and RSV bronchiolitis on the development of asthma up to age 7 1 / 2 is shown in Figure 2. Among the 21 children with both asthma heredity and RSV bronchiolitis, eight children (38%) developed asthma compared with none of those with a family history of asthma but no RSV bronchiolitis. Among the 30 children with a family history of atopy and RSV bronchiolitis, 14 (47%) were sensitized compared with 15 children (26%) out of the 57 with a positive family history but without bronchiolitis (NS, p 0.093, RR 1.773, 95% CI to 3.164). Several possible hereditary and environmental risk factors for the development of different bronchial obstructive symptoms and allergic sensitization were evaluated using Fisher exact test (Table 4). RSV bronchiolitis was the most important risk factor for asthma and also an important risk factor for any wheezing and allergic sensitization. Male sex was the most important risk factor for any wheezing. The most important risk factor for allergic sensitization was a double parental history of atopy. An inverse relationship was seen between the occurrence of indoor furred animals and allergic sensitization. The risk factors in Table 4 with a p value 0.10 were entered in forward stepwise logistic regression analyses to estimate which factors were independently related to the development of obstructive symptoms and sensitization (Table 5). RSV bronchiolitis was an independent risk factor for bronchial obstructive symptoms. Male sex was the most important risk factor for any wheezing. A family history of asthma in one or two parents and the occurrence of indoor furred animals were also important for any wheezing. A double parental history of atopy was the most important risk factor for allergic sensitization, followed by an RSV bronchiolitis. DISCUSSION The results of this follow-up at age 7 1 / 2 of children with RSV bronchiolitis in infancy and a matched control group show: (1) a pronounced higher rate of bronchial obstructive disease in Figure 1. The course of bronchial obstructive disease and allergic sensitization to inhaled allergens up to age 7 1 / 2 in children with RSV bronchiolitis in infancy and a control group. (a) Rates of current asthma and current any wheezing in the 47 RSV children and the 93 control children, at ages 1, 3, and 7 1 / 2. (b) Rates of a positive Phadiatop and a positive SPT to common inhaled allergens in 44 RSV children, 87 control children (Phadiatop), and 89 control children (SPT) at ages 3 and 7 1 / 2.

4 1504 AMERICAN JOURNAL OF RESPIRATORY AND CRITICAL CARE MEDICINE VOL TABLE 3 NUMBER (PERCENTAGE) OF CHILDREN WITH POSITIVE SPT AND SERUM IgE ANTIBODIES TO FOOD AND INHALANT ALLERGENS IN THE RSV GROUP AND THE CONTROL GROUP AT AGE 7 1 / 2 Allergy Test RSV Group Control Group p Value RR 95% CI Any positive test 18/44 (41%) 19/86 (22%) Any positive inhalant test 15/44 (34%) 13/87 (15%) Positive SPT, total 9/44 (20%) 5/89 (6%) Food 0/44 0/89 Inhalants 9/44 (20%) 5/89 (6%) Positive serum test, total 17/44 (39%) 18/87 (21%) Food 9/44 (20%) 10/87 (11%) Inhalants 14/44 (32%) 12/87 (14%) the RSV children and a higher rate of atopic asthma, i.e., asthma with allergic sensitization; (2) a higher rate of clinical allergy to inhaled allergens in the RSV children, judged by the higher rate of ARC; (3) a higher rate of allergic sensitization to inhaled allergens in the RSV children; (4) no tendency for the rates of bronchial obstructive disease and sensitization among the control subjects to catch up with the rates in the RSV group; (5) that RSV bronchiolitis was a more important risk factor for the development of asthma up to age 7 1 / 2 than heredity for atopy/asthma, male sex, or various environmental factors; (6) that RSV bronchiolitis was an independent risk factor for allergic sensitization at age 7 1 / 2. To our knowledge, the present study with a control group is the only prospective one in which all the index children had their infection verified by an RSV test and in which antibody response to RSV was checked in both index children and control subjects. The study is the only truly prospective one so far in that the control subjects were recruited concomitantly with the index children during infancy. The procedure for selecting the control subjects was aimed at guaranteeing similarity regarding sex, age, and also geographical and social environment. No significant differences in various hereditary and environmental factors between the groups were found, neither in the previous nor in the present analysis. The fact that there were no dropouts in the three follow-ups makes the data obtained valid. The children with RSV bronchiolitis constituted a homogeneous group from the same epidemic admitted to the same hospital; 91% were 6 mo of age and for all but one of them it was their first episode of bronchial obstructive symptoms (7). In a recent study from the Tucson Study Group male Figure 2. Influence of a family history of asthma and RSV bronchiolitis on the development of asthma up to age 7 1 / 2 in 140 children, 47 of whom had RSV bronchiolitis in infancy. gender was a risk factor for RSV lower respiratory tract infection in Hispanic children but not in children with Anglo maternal ethnicity (17). The frequency of boys in the RSV bronchiolitis group in the present study was 45% (95% CI 31 to 59%) or 48% (95% CI 35 to 62%) of all 52 eligible children which agrees with other studies, e.g., the ones by Pullan and Hey, 50% (2) and Murray and coworkers, 53% (5) and does not differ significantly (p 0.24) from the frequency (57% of 1,563 randomly selected cases) in a large multicenter study published in 1998 of infants younger than 12 mo hospitalized with RSV infections (1). Maternal smoking was not a risk factor for lower respiratory tract infection in the Tucson study (17) and no difference in smoking habits in the family was found between the index group and the control subjects neither in the study by Murray and coworkers (5) nor in the present study. It has been shown in a Swedish study that after two RSV seasons approximately three-quarters of tested children had IgG antibodies against RSV (18). As all children in the present study were exposed to the same RSV epidemic, RSV IgG antibodies were measured at age 1 in both groups and the difference was highly significant, 89% in the RSV group versus 27% of the control subjects (7). All of the 24 control subjects with RSV IgG antibodies had a history of a mild respiratory tract infection at the time of the RSV epidemic (in seven cases combined with a slight wheezing not requiring a visit to a doctor). When also IgA antibodies were analyzed, all children in the RSV group were seropositive compared with 42% of the control subjects (p 0.001) (19). These findings underline the fact that a severe RSV infection was an important difference between the groups. The results of the present study and of many other studies of index children using comparison groups have delivered unambiguous evidence that RSV bronchiolitis in infancy entails a higher risk of development of future bronchial obstructive disease although the connection with atopic asthma is not known (2 7). A finding to emphasize in the present study was that RSV bronchiolitis had by far the highest independent risk ratio for asthma in the multivariate analysis of the combined group of 140 children. The results of various previous studies concerning the risk of developing IgE-mediated allergy in connection with RSV bronchiolitis are not as unambiguous as the results indicating an increased risk of asthma. A mild or subclinical RSV infection does not seem to be related to an increased risk for sensitization. Thus, in a German study only a weak connection with allergic sensitization during infancy but not later, was found in a large group of children with IgG antibodies to RSV (20). This agrees with the findings both in our previous study (7) and in the present one, inasmuch as no increased risk for sensitization was seen either at age 3 (7) or at age 7 1 / 2 in the 24

5 Sigurs, Bjarnason, Sigurbergsson, et al.: RSV and Development of Asthma and Allergy 1505 TABLE 4 UNIVARIATE TEST OF VARIOUS POSSIBLE RISK FACTORS FOR CUMULATIVE BRONCHIAL OBSTRUCTIVE SYMPTOMS AND ALLERGIC SENSITIZATION AT AGE 7 1 / 2 USING FISHER EXACT TEST IN 140 CHILDREN, 47 OF WHOM HAD RSV BRONCHIOLITIS* Asthma Any Wheezing Sensitization Risk Factor p Value OR 95% CI p Value OR 95% CI p Value OR 95% CI RSV bronchiolitis ( ) ( ) ( ) Heredity for atopy, total ( ) ( ) ( ) Heredity for atopy, parent(s) ( ) ( ) ( ) Double heredity for atopy, total ( ) ( ) ( ) Double heredity for atopy, parents ( ) ( ) ( ) Heredity for asthma, total ( ) ( ) ( ) Heredity for asthma, parents ( ) ( ) ( ) Double heredity for asthma, total ( ) ( ) ( ) Double heredity for asthma, parents NA (NA) ( ) ( ) Male sex ( ) ( ) ( ) Indoor furred animals, cumulative ( ) ( ) ( ) Smoking in family, cumulative ( ) ( ) ( ) Definition of abbreviations: 95% CI 95% confidence intervals; NA not applicable; OR odds ratios. * Significant values are italicized. Heredity, total means disease in one or two parent(s) and/or sibling(s). control children with IgG antibodies to RSV at age 1, and is also in line with a recent publication from the Tucson Study Group, where no connection to atopy was found in children with RSV infection not requiring hospitalization (21). To our knowledge there are four previous studies with control groups evaluating allergic sensitization by SPT in children with RSV and unspecified bronchiolitis severe enough to cause hospitalization in infancy (2 6). No difference in sensitization was seen at age 8 in 1981 (3) or at age 2 in 1987 (4) whereas a third study in 1982 found fewer positive SPT in index children in comparison to control subjects (2). In the fourth study sensitization was significantly increased in the bronchiolitis group at age 6 in 1992 in 73 index children compared with 73 control subjects, but not age 9 to 10 in 1997 in 61 index children versus 47 control subjects (5, 6). The results to emphasize in the present study are the findings of an increased risk of allergic sensitization estimated by both SPT and serum IgE antibodies both at age 3 and 7 1 / 2 and of atopic asthma and clinical allergies in the RSV group at age 7 1 / 2 in comparison to the control group in spite of a similar family history. It raises the possibility that RSV bronchiolitis severe enough to cause hospitalization per se can make the children more prone to develop atopic allergy afterwards. The evaluation of other possible risk factors beside RSV bronchiolitis for the development of different bronchial obstructive symptoms and allergic sensitization in the whole group of 140 children showed that male sex as well as a family history of asthma were also significant risk factors for any wheezing in agreement with other studies of wheezing in childhood (e.g., References 22, 23 and 23, 24, respectively). Smoking in the family was not associated with an increased risk of bronchial obstructive symptoms at this follow-up in contrast to the follow-up at age 3 (7) which agrees with the study by Murray and coworkers in which the follow-up at age 5 showed a slight connection between wheezing and smoking in the family whereas the one at age 9 to 10 did not (5, 6). Thus, tobacco smoke exposure is probably more important for wheezing in the younger age groups. Not surprisingly, the most important risk factor for allergic sensitization was a double parental history of atopy in agreement with many previous studies on sensitization (e.g., 23). There are two major hypotheses concerning RSV and subsequent bronchial obstructive symptoms and allergy, as discussed, for example, by Long and coworkers (8). One hypothesis proposes that RSV (and perhaps other viral infections) early in life may cause later bronchial obstructive symptoms by damaging the growing lung or by altering host immune responses (8). The second hypothesis asserts that RSV infection is more severe in infants with some underlying predisposition, which per se contributes to later bronchial obstructive disease (10, 11). These two hypotheses are not mutually exclusive, and it is possible that some preexisting condition in combination with the viral infection contributes to the long-term respiratory symptoms (8). It does not seem likely that heredity for atopy/asthma is the reason why some children develop RSV bronchiolitis and some no, or only mild, symptoms after contact with RSV, as the family history of atopy/asthma was similar in the study groups in this as well as other studies using comparison groups (2 7). Another possible reason for the development of RSV bronchiolitis may be a preexisting diminished lung function which could also increase the risk of wheezing in connection with other viruses (24, 25). It has been shown in animal studies that viral infections can increase the risk of allergic sensitization (26, 27). Nearly 20 yr TABLE 5 MULTIVARIATE TEST OF POSSIBLE RISK FACTORS FOR CUMULATIVE BRONCHIAL OBSTRUCTIVE SYMPTOMS AND ALLERGIC SENSITIZATION AT AGE 7 1 / 2 BY A STEPWISE LOGISTIC PROCEDURE IN 140 CHILDREN, 47 OF WHOM HAD RSV BRONCHIOLITIS* Risk Factor Asthma Any Wheezing Sensitization RSV bronchiolitis (12.7, ) (5.3, ) (2.4, ) Double heredity for n.s atopy, parents (5.6, ) Heredity for asthma, NS parent(s) (3.0, ) Male sex (4.4, ) Indoor furred animals, NS cumulative (2.9, ) Definition of abbreviation: NS not significant. * Risk factors with a p value 0.10 in Fisher exact test (see Table 4) were entered in the model. Risk factors marked with were not qualified to be included. Only risk factors with significance in at least one of the outcome variables are shown, with p values (odds ratio, 95% CI).

6 1506 AMERICAN JOURNAL OF RESPIRATORY AND CRITICAL CARE MEDICINE VOL ago, it was suggested that viral infections could increase the risk of allergy in infants (28) and, in recent years, the connection between viral infections and atopic disease has been much discussed (11, 29). A viral influence causing a switch from T helper cell, type 2 (Th2) to Th1 lymphocytes, and thus a lower production of IgE antibodies, has been suggested (29), but a more complex mechanism may operate in RSV infection, judged by animal studies (30, 31) and by the results of the present and other clinical studies (7, 32 34). It has been hypothesised that an early RSV infection may perpetuate the Th2 dominance seen in the fetus (9). Our previous study of the immune responses to RSV at age 1 in the present study groups showed that the children with RSV bronchiolitis tended to have higher IgA antibody titers than the control children and that the highest titers were found in the children who later developed IgE antibodies (19). Because both IgA and IgE antibody formation are strongly Th2 cell dependent, these findings are also compatible with the theory that RSV may increase the risk of allergy (19). The fact that RSV was an independent risk factor both for asthma and for allergic sensitization in the multivariate test in the whole study group suggests that severe RSV bronchiolitis in and of itself can contribute to the later bronchial obstructive symptoms and to the allergic sensitization. At both the previous follow-up and the present one, however, it was evident that a family history of asthma without bronchiolitis carried a significantly lower risk of asthma than a combination of these factors. It is likely that two kinds of wheezing may follow RSV bronchiolitis, one more benign, which subsides with time, similarly to other kinds of bronchial obstructive symptoms in early childhood (11, 24), and one more severe, which may be accentuated by heredity for atopy/asthma (7, 35). To decide which of the hypotheses concerning RSV and later asthma/allergy is the most relevant, it is necessary to conduct prospective, randomized studies, including some form of intervention against RSV, such as vaccination or medication against the virus (8). Descriptive studies like the present one can only suggest which connections between RSV and subsequent symptoms seem most likely. In conclusion, RSV bronchiolitis in infancy was strongly associated with both asthma as well as other forms of bronchial obstructive disease and allergic sensitization at age 7 1 / 2. There was no tendency for the frequencies of asthma and sensitization among the control children to catch up with those in the RSV group. The fact that RSV bronchiolitis had a substantially higher risk ratio for future asthma than a family history of atopy/asthma and also a significantly elevated risk ratio for allergic sensitization supports the theory that RSV may influence the mechanisms involved in the development of asthma and allergy in children. Acknowledgment : The authors thank Professor Peter Openshaw of the Department of Respiratory Medicine at St. Mary s Hospital, London for fruitful discussions. References 1. Behrendt, C. E., M. D. Decker, D. J. Burch, and P. H. Watson for the International Study Group International variation in the management of infants hospitalized with respiratory syncytial virus. Eur. J. Pediatr. 157: Pullan, C. R., and E. N. Hey Wheezing, asthma and pulmonary dysfunction 10 years after infection with respiratory syncytial virus. B.M.J. 284: Sims, D. G., P. S. Gardner, D. Weightman, M. W. Turner, and J. F. 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