Recovery of Drug-Resistant Influenza A Virus during Therapeutic Use of Rimantadine

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1 ANTIMICROBIAL AGENTS AND CHEMOTHERAPY, Sept. 1991, p /91/ $02.00/0 Copyright D 1991, American Society for Microbiology Vol. 35, No. 9 Recovery of Drug-Resistant Influenza A Virus during Therapeutic Use of Rimantadine FREDERICK G. HAYDEN,'* STEVEN J. SPERBER,lt ROBERT B. BELSHE,2t RICHARD D. CLOVER,3 ALAN J. HAY,4 AND STEPHEN PYKE4 University of Virginia, Charlottesville, Virginia ; Marshall University, Huntington, West Virginia ; Oklahoma University, Oklahoma City, Oklahoma ; and National Institute for Medical Research, London, United Kingdom4 Received 26 December 1990/Accepted 24 June 1991 The therapeutic activity of rimantadine and its relationship to the shedding of drug-resistant influenza A virus were assessed in two randomized, double-blind, placebo-controlled trials involving patients with laboratory-documented influenza A virus (H3N2 subtype) illness of 2 days' duration or less. In a family-based study, rimantadine treatment for 10 days (24 children and adults) was associated with significant decreases in the number of days to a 50% reduction in symptoms (mean difference, 2.5 days), days of fever (1.6 days), and days of restricted activity (1.5 days) compared with the results obtained with placebo-treated patients (32 children and adults). Drug-resistant virus was recovered from eight (33%) of the rimantadine recipients on day 5. No differences in patient demographics or illness severity at the time of enrollment in the study were apparent between those who shed resistant virus and those who did not. Illness resolution tended to be slower in those who shed resistant virus compared with that in those who did not. In a study of adults treated for 5 days (six treated with rimantadine, six treated with placebo), resistant virus was recovered in three rimantadine recipients by day 3 of treatment. The results indicate that drug-resistant influenza A virus (H3N2) can be recovered from rimantadine-treated children and adults as early as 2 days after starting treatment, but that rimantadine retains a net therapeutic benefit compared with that of placebo. Amantadine and rimantadine have been shown to be effective prophylaxis for naturally occurring influenza A virus infections in children and adults (6-10). In contrast to vaccines, these drugs have also been found to be therapeutically active in patients with uncomplicated acute illness (6, 9, 12, 15, 18, 21, 23). When therapy is begun within 48 h of symptom onset, 1- to 2-day reductions in the duration of fever and illness and, in some trials (9, 12, 15, 21), a decreased frequency of viral shedding have been observed. In adults, the drugs have comparable therapeutic effects at conventional doses, although rimantadine is associated with a lower risk of side effects involving the central nervous system (9, 14, 21). Rimantadine has been shown to be effective in elderly patients with acute influenza (6) and more effective than acetaminophen in children with illness caused by infection with the influenza A virus H3N2 subtype (12) but not the HlNl subtype (20). Those studies found initial decreases in viral titers but similar or higher frequencies of viral shedding in rimantadine recipients later in the course of therapy (6, 8, 12, 20). Pediatric studies have documented the recovery of drugresistant virus from treated children (3, 4, 12, 20), and this finding has been recently confirmed in adults receiving rimantadine (13) or amantadine (17, 19). The transmission of drug-resistant virus from treated patients to contacts receiv- * Corresponding author. t Present address: Division of Allergy, Immunology, and Infectious Diseases, Robert Wood Johnson Medical School, University of Medicine and Dentistry of New Jersey, New Brunswick, NJ t Present address: Division of Infectious Diseases, School of Medicine, St. Louis University, St. Louis, MO Present address: School of Medicine, University of Texas, Galveston, TX ing drug prophylaxis has been found in the household setting during rimantadine use (3, 13) and in nursing home outbreaks managed with amantadine (17, 19). However, limited data are available to determine the frequency, kinetics, and clinical significance of recovering drug-resistant virus from treated patients. We conducted two clinical trials, one in families and one in ambulatory adults, to address these issues and to reassess the therapeutic efficacy of rimantadine. MATERIALS AND METHODS Study design. Both clinical trials were randomized, placebo-controlled, and double-blind in design. The family-based study (13) was conducted at three centers (Charlottesville, Va.; Huntington, W.V.; Oklahoma City, Okla.), whereas the adult treatment study was undertaken in Charlottesville, Va. Both studies were conducted during the winter season of 1988, during which influenza A virus (H3N2 subtype) caused outbreaks. Written informed consent in a form approved by the respective institutional review boards of the participating centers was obtained. Compensation was provided to participants. Family-based study. The methods used for recruitment, surveillance, sampling, and drug administration in the multicenter, family-based trial have been described previously (13). Briefly, when an influenzal illness developed in one or more family members, all eligible family members (ages, 1 to 75 years), including healthy contacts and those who were ill, were assigned as a block to receive either rimantadine or placebo once daily for 10 days. Rimantadine was administered in tablet or syrup form (for children <10 years of age). The daily dose was 200 mg for adults and children 10 years and older. The dose was 5 mg/kg of body weight up to a

2 1742 HAYDEN ET AL. ANTIMICROB. AGENTS CHEMOTHER. TABLE 1. Demographic characteristics of febrile influenza A virus (H3N2)-infected subjects treated with rimantadine or placebo Treatment group Rimantadine-treated subgroup Factor Placebo Rimantadine No resistant virus Resistant virus (n = 32) (n = 24) (n = 16) (n = 8) Age (no.) 1-9 yr yr yr Age (yr [mean ± SD]) 15 ± ± ± ± 16 Female (no.) Male (no.) Location (no.) Oklahoma West Virginia Virginia Race (no.) Black Caucasian Smokers (no.) Culture positive on enrollment (no.) Interval to treatment (no.) day day day Day 1 symptom score (mean + SD) 12 ± 7 16 ± ± ± 6 Day 1 temperature ( F [mean ± SD]) ± 1.2a ± ± ± 0.9 a p < 0.01, rimantadine versus placebo. maximum of 150 mg/day for those 9 years old and younger or those who weighed less than 30 kg. In this report we describe in detail the outcomes observed in the index and coprimary cases with laboratory-documented influenza A virus (H3N2) illness who began treatment within 2 days of illness onset. The occurrence of respiratory and systemic symptoms and oral temperature were recorded once daily during the treatment period. The severity of symptoms was rated on a four-point scale (0 to 3, absent to severe). Lost days of school or work time, confinement to bed, physician visits, and days of restricted activity were also recorded. Whenever possible, the households were visited on treatment days 5 and 10 to collect follow-up specimens for virus isolation. Adult treatment study. Previously healthy adults with acute febrile influenzalike illness of less than 2 days' duration were recruited through the University of Virginia Student Health Service or Employee Health Service. As described previously (5), rimantadine-treated patients were given a single loading dose of 400 mg at the time of enrollment into the study followed by once-daily doses of 200 mg on treatment days 2 through 5. Controls received an identically appearing placebo throughout the 5-day treatment period. To ensure compliance, doses were administered under the direct observation of the study staff. Participants were either admitted to the Health Sciences Center's Clinical Research Center or visited by study staff at their residences, so that nasal washings for virus isolation could be collected twice daily during the first 2 days of treatment and once daily for 3 additional days. Temperature and symptom recordings were done as described above. Virologic studies. Virus isolation from respiratory tract specimens was done on monolayers of Madin-Darby canine kidney (MDCK) cells, primary rhesus monkey kidney cells, or both. Titers were determined by inoculating serial log1o dilutions of once frozen (-70 C) nasal washings onto quadruplicate MDCK cell monolayers. First-passage isolates were stored at -70 C for testing of their susceptibility to rimantadine by previously described enzyme-linked immunoassays (3, 4, 13). Resistant isolates, defined by an inhibitory concentration of >1.0,ug of rimantadine per ml for the isolates, were confirmed by nucleotide sequence analysis of the portion of RNA segment 7 coding for the transmembrane region of the M2 protein (4, 13). Evaluation of therapeutic efficacy. Infection was defined as the isolation of virus from respiratory secretions, a fourfold or greater rise in hemagglutination-inhibition or neutralizing antibody titers in serum, or both. A febrile influenzalike illness was defined as the presence of fever (oral temperature,.38.00c or F) and at least one of the following for at least 1 day: cough, sore throat, nasal obstruction, chest pain, myalgia, headache, runny nose, fatigue, and malaise. In the family-based study, symptom scores were generated for each study day by totaling the severity rating (0 to 3) for the following symptoms: fever, chills, headache, muscle aches, runny nose, stuffy nose, sneezing, sore or scratchy throat, hoarseness, cough, nausea, vomiting, dizziness, diarrhea, chest pain, fatigue, and malaise. Beginning with the

3 VOL. 35, 1991 RIMANTADINE TREATMENT AND DRUG-RESISTANT INFLUENZA A 1743 TABLE 2. Clinical outcomes in influenza virus A (H3N2)-infected patients treated with ramantadine or placebo according to time of treatmenta Days: Onset of No. of treatment and To 50% Of missed Of restricted physician group reduction in Of fever school or work In bed activity visits symptom score Days 0-2 Placebo 6.5 ± 2.8b (32) 3.3 ± 2.5- (30) 3.2 ± 1.5d (29) 2.3 ± 1.7 (32) 3.8 ± 2.4d (32) 0.4 ± 0.8 (32) Rimantadine 4.0 ± 1.9 (24) 1.7 ± 1.0 (21) 2.2 ± 1.8 (20) 1.6 ± 1.6 (23) 2.3 ± 2.1 (23) 0.3 ± 0.6 (23) Day 0 or 1 Placebo 6.8 ± 2.7b (26) 3.6 ± 2.5c (26) d (24) 2.5 ± 1.8 (26) 4.3 ± 2.2d (26) 0.4 ± 0.9 (26) Rimantadine 3.9 ± 1.9 (19) 1.8 ± 1.1 (17) 2.2 ± 1.9 (16) 1.6 ± 1.5 (18) 2.5 ± 2.2 (18) 0.3 ± 0.7 (18) Day 0 Placebo 6.9 ± 2.8c (16) 4.1 ± 2.7d (16) 3.3 ± 1.1 (14) 2.4 ± 1.4 (16) 4.2 ± 2.1 (16) 0.3 ± 0.4 (16) Rimantadine 3.4 ± 1.3 (7) 2.0 ± 0.6 (6) 1.8 ± 1.3 (5) 1.3 ± 1.2 (6) 1.8 ± 1.7 (6) 0.5 ± 0.8 (6) a Values are means ± standard deviations. Values in parentheses are the number of evaluable subjects. b p < for rimantadine versus placebo by the two-sample Wilcoxon test. C p < d p < o0o5. first day of treatment, the number of days to a sustained reduction of 50% or more in the symptom score and the number of days to a sustained afebrile condition were determined for each subject. Functional outcomes, including overall days of restricted activity, number of physician visits, and use of supplemental medications, were taken from the symptom records. Statistical analysis. Assessment of possible demographic differences between treatment subgroups was carried out by use of the Fisher exact test, in the case of variables on two levels, and the chi-square test for independence, in the case of variables on more than two levels. The importance of treatment group in regard to the number of days of symptoms, fever, and functional limitation was assessed in two ways. Initially, by the method of analysis of variance, the significance of treatment was assessed in the presence of other known possible explanatory covariates (age, sex, etc.), in order to take account of possible correlations between these covariates. Since this takes no account of the nonnormal distribution of many of these measures, when the results for the treatment group were found to be significant (P < 0.05), the two-sample Wilcoxon rank sum test of equal medians (adjusted for ties) was used to provide a measure of significance free from distributional assumptions. Comparisons of daily symptom scores and temperatures was by the t test, and P values given for treatment group differences are for the two-tailed test of equal means. Because of multiple comparisons between the groups over time, P values of were considered significant. RESULTS Family-based study. (i) Participants. A total of 44 index or coprimary cases receiving placebo and 27 receiving rimantadine had laboratory evidence of influenza A virus (H3N2) infection. Of these patients, fever was documented in 32 who received placebo (73%) and 24 who received rimantadine (89%). The presence of fever was associated with a higher illness burden, as reflected in significantly (P < 0.01) more days of missed work or school, disabling illness requiring bedrest, and restricted activity (data not shown). In order to eliminate the influence of this variable, subsequent analysis of treatment effects focused on those individuals who had a documented fever. The febrile group used for analysis comprised 56 patients (32 who received placebo, 24 who received rimantadine) who began treatment within 2 days of symptom onset. The treatment groups were balanced with respect to age, gender, location, and other demographic characteristics (Table 1). The interval from the onset of symptoms to the initiation of therapy and initial symptom scores did not differ significantly between the groups, although the initial temperature was significantly higher in rimantadine recipients compared with that in placebo recipients (Table 1). (ii) Evaluation of therapeutic efficacy. For all subjects (Table 2), the number of days to a 50% or greater reduction in symptom score was reduced by a mean of 2.5 days in rimantadine recipients compared with that in placebo recipients (P < 0.005). Similarly, the number of days of fever was reduced by an average of 1.6 days (P < 0.01), the number of s y M pt 0 S C 0 RE 2Or 15 ' 10 F 5 N GL -0- PLACEBO (N-az -6- NO RESISTANT (N-16) -6- RESISTANT (N-B), -GL,, IG _ -G. - o TREATMENT DAY FIG. 1. Resolution of symptoms in rimantadine-treated, influenza A virus (H3N2)-infected patients who shed resistant virus (n = 8) and those who did not have detectable shedding of resistant virus (n = 16). No significant differences existed between the two groups. The symptoms scores in the placebo group (n = 32) are indicated for comparison. -0

4 1744 HAYDEN ET AL. TABLE 3. Concomitant medication use during treatment with rimantadine or placebo % of subjects in the 5 of treatment days Type of following groups: for subjects in the medication following groups: Placebo Rimantadine Placebo Rimantadine Acetaminophen Aspirin, NSAIDS' Antitussives Antibiotics Otherb "NSAIDS, nonsteroidal antiinflammatory agents. b Decongestants, antihistamines, and throat lozenges or sprays. missed days of work or school was reduced by an average of 1.0 days (P < 0.05), and the number of overall days of restricted activity was reduced by an average of 1.5 days (P < 0.05). Days ill in bed and the number of physician visits did not differ between groups. When only those individuals who initiated treatment on the same day as symptom onset (day 0) or within 1 day of symptom onset were considered (day 0 or 1), similar differences in favor of rimantadine treatment were found (Table 2). Analysis of symptom and temperature data on a daily basis reinforced these findings. Placebo recipients had a peak in symptom scores on days 2 and 3 of treatment, with a gradual decline thereafter (Fig. 1). In contrast, rimantadine recipients had a steady decline in symptom scores from the time of treatment, and symptom scores on treatment days 3 to 9 differed significantly (P 0.01) from those of the placebo group. Rimantadine recipients also had more rapid resolution of fever, and mean temperatures differed significantly from those of the placebo recipients on treatment days 3 and 4 (data not shown). The use of concomitant medications which might have influenced illness resolution was common and comparable in the two groups (Table 3). Overall, such medications were used by 75% of rimantadine recipients and 72% of placebo recipients. The most commonly used medication was acetaminophen (46% of rimantadine recipients, 47% of placebo recipients), which was ingested on days (mean ± standard deviation) by rimantadine recipients and on 2.0 ± 2.8 days by placebo recipients. Antitussives tended to be used on a higher portion of treatment days in the placebo group (1.4 ± 2.5 days) than in the rimantadine group (0.5 ± 1.2 days) (P > 0.1). Antibiotic use did not differ significantly between the groups. (iii) Effect of shedding resistant virus. Influenza virus was recovered from 9 of 31 placebo recipients (29%) and 10 of 24 rimantadine recipients (42%) from whom samples for culture ANTIMICROB. AGENTS CHEMOTHER. were obtained on treatment day 5 (not significant). Cultures collected on treatment day 10 were positive in 3 of 31 placebo recipients (10%) and 0 of 24 rimantadine-treated patients. Eight rimantadine recipients (33%) and no placebo recipients were found to shed resistant virus during therapy. In rimantadine recipients, 80% of isolates obtained on day 5 were resistant. The patterns of the virus shedding and the M2 protein changes correlated with resistance development (one at position 27 [Val to Ala], two at position 30 [Ala to Thr, Ala to Val], five at position 31 [Ser to Asn]) have been described in detail previously (13). No significant differences in patient characteristics were found between rimantadine recipients who shed resistant virus and those who did not (Table 1). Those who shed resistant virus included both children and adults. They were not treated earlier in the course of their illness, compared with those who did not shed resistant virus, nor did they have more severe illness at the time of enrollment, as reflected in higher symptom scores or oral temperatures (Table 1). Resolution of illness tended to be slower in the rimantadine subgroup that shed resistant virus (Table 4). Days of fever were fewer by an average of 1.1 days (P < 0.05), and other measures of functional impairment tended to be reduced to a greater extent in rimantadine recipients who did not shed resistant virus compared with reductions in those who did (Table 4). The number of days to 50% reduction in symptom scores was less by a mean of 1.7 days in those who did not shed resistant virus compared with that in those who did (0.05 < P < 0.1). When symptom scores were analyzed on a daily basis (Fig. 1), no significant differences existed between the two rimantadine groups. Scores tended to decrease more slowly in the subgroup that shed resistant virus (Fig. 1), but the average scores in this group continued to decline on treatment days 2 and 3, when the scores in the placebo group were highest. Analysis of fever resolution on a daily basis also found that the average temperatures did not differ between the rimantadine subgroups over the first 4 days of treatment (Fig. 2). A nonsignificant elevation of temperature appeared in the rimantadine subgroup that shed resistant virus on treatment day 5, but it promptly disappeared thereafter. Adult treatment study. The small number of patients with laboratory-documented influenza A virus illness (six placebo recipients, six rimantadine recipients) was insufficient to provide an adequate assessment of clinical efficacy. Trends toward reductions in days to 50% symptom improvement and days of restricted activity, each averaging 1.5 days, were apparent in the rimantadine group compared with those in the placebo group. Virus was isolated from all of these patients on the first treatment day, and patients were found to have fourfold or greater rises in serum hemagglutination inhibition antibody titers. Quantitation of virus that was TABLE 4. Clinical outcomes in febrile influenza virus A (H3N2)-infected patients treated with rimantadine and their relationships to shedding of drug-resistant virus Days': Group To 50% reduction Of fever Of missed I bed Of restricted in symptom score school or work n e activity No resistant virus 3.4 ± 1.6 (16) 1.3 ± 0.5 (13) 1.7 ± 1.3 (14) 1.3 ± 1.3 (16) 1.8 ± 1.5 (16) Resistant virus 5.1 ± 2.2b (8) 2.4 ± 1.3c (8) 3.3 ± 2.4 (6) 2.3 ± 2.0 (7) 3.7 ± 2.7 (7) a Values are means ± standard deviations. Values in parentheses are the number of evaluable subjects. b 0.05 < P < 0.1 for resistant virus versus no resistant virus by the two-sample Wilcoxon test. P < 0.05.

5 VOL. 35, 1991 RIMANTADINE TREATMENT AND DRUG-RESISTANT INFLUENZA A PLACEBO (N-3 --Qe-.V DOITkfM _ (N-1uJ R RESSTANT (NS) A L T E M P 100 A T u R 9 E TREATMENT DAY FIG. 2. Resolution of fever in rimantadine-treated, influenza A virus (H3N2)-infected patients in whom shedding of drug-resistant virus was detected (n = 8) compared with that in those in whom resistant virus was not found (n = 16). No significant differences existed between the two groups. The placebo group is indicated for comparison. Oral temperatures are in degrees Fahrenheit. recoverable in nasal washings found no differences between the groups on the first treatment day (Table 5). Placebo recipients continued to shed relatively high titers of virus through treatment day 4. In contrast, rimantadine recipients had a prompt decline in viral titers by the morning of the second treatment day and had low titers thereafter (means, <1.0 log1o 50% tissue culture infective dose [TCID50] per 0.2 ml of nasal washing). Three of six rimantadine recipients (50%) and none of the placebo recipients shed resistant virus on one or more treatment days (Fig. 3). Sequence analysis of pairs of susceptible and resistant viruses showed that two had changes at position 31 (Ser to Asn) and one had a change at position 30 (Ala to Val). The proportion of subjects shedding virus tended to be lower in rimantadine recipients during the later half of the treatment period, but the small numbers of subject precluded reliable statistical analysis. Resistant virus was first detected in one rimantadine recipient on the evening of the second treatment day and was present in all samples that yielded the virus from rimantadine-treated patients on treatment days 3 to 5 (Fig. 3). The maximal virus titers observed on the first treatment day tended to be higher in rimantadine recipients who shed resistant virus (mean ± standard deviation, 4.1 ± 1.0 log1o TCID50I0.2 ml) compared with that in those who did not (3.1 ± 2.0 log1o TCID5J/0.2 ml), but overlap existed between these subgroups. 01.5t 0CD z am pm am pm Theatment Day FIG. 3. Frequency of virus recovery in influenza A virus (H3N2)-infected adults treated with rimantadine (n = 6) or placebo (n = 6). The recovery of drug-susceptible (E) or -resistant (-) virus in rimantadine recipients is indicated. O, placebo. DISCUSSION These studies enabled an assessment of the frequency and possible clinical importance of recovering drug-resistant virus during therapeutic use of rimantadine. The findings confirm that drug-resistant influenza A virus can be recovered from children or adults treated with rimantadine (3, 4, 12, 13). The family-based trial found that resistant virus was recovered from samples from 33% of rimantadine-treated patients collected on treatment day 5. Similarly, one placebo-controlled trial of H3N2 subtype-infected children found that resistant virus was recovered from 27% of all rimantadine recipients, generally beginning on days 4 to 6 after starting therapy, and from 45% of those who shed virus up to day 7 (12). The adult treatment study, which incorporated frequent samplings for virus isolation, detected resistant virus from three of six rimantadine recipients and found that shedding can begin as early as treatment day 2. Rimantadine recipients from whom virus was recovered on day 4 or 5 of treatment were likely to shed resistant virus in both the family-based (80%) and adult treatment (100%) studies. The adult treatment study confirmed earlier observations in children (12) that once shedding of resistant virus develops, subsequent isolates from the same patient are also resistant. However, the limited duration of follow-up in the present trial did not allow an assessment of the duration of shedding of resistant virus. In the family-based study, patients who shed resistant virus on treatment day 5 were culture negative on day 10. The clinical importance of recovering drug-resistant virus from treated patients is incompletely resolved. In our adult treatment study, rimantadine administration was associated with significant reductions in virus titers, despite the recov- Treatment TABLE 5. Comparison of influenza virus A titers in nasal washings of adults treated with rimantadine or placebo Virus titers (log10 TCID5W/0.2 ml of nasal wash) on treatment day"': Enrollment 1 (p.m.) 2 (a.m.) 2 (p.m.) Rimantadine (n = 6) 3.7 ± 1.7b 2.5 ± ± ± 1.3b 0.7 ± ± ± 1.3 Placebo (n = 6) 2.9 ± ± 1.2b 3.3 ± 1.3' 2.4 ± ± 2.1d 2.2 ± ± 1.5 a Values are means + standard deviations. For calculation purposes, negative samples were assigned a value of -0.5 log1o TCID5J0.2 ml. b Five samples only. c P < 0.05 versus rimantadine. d 0.05 < P < 0.10 versus rimantadine.

6 1746 HAYDEN ET AL. ery of resistant virus from some patients. One earlier trial (12) found that in rimantadine-treated children, the frequencies and amounts of virus recovered after therapy were higher compared with those recovered from a placebo group. In contrast, no overall increase in the frequency or quantity of virus recovery was evident in the rimantadine-treated adults compared with that in the placebo group in this study. This finding confirms earlier observations in young (15) and elderly (6, 8) adults who received rimantadine for the treatment of uncomplicated influenza. These age-related differences may relate to the effects of rimantadine therapy on virus replication and indirectly on influenza-specific cellular immune responses (12, 16). However, patients who remain virus positive after several days of treatment appear to have a high likelihood of shedding resistant virus, and one of our patients continued to shed high titers of resistant virus during therapy (2.50 log1o TCID50/0.2 ml) on treatment day 5. Under appropriate epidemiologic circumstances, such individuals may transmit resistant virus to close contacts. This problem has been documented in households during rimantadine use (13) and in a nursing home during amantadine use (17, 19). Unfortunately, the current studies did not identify any obvious clinical or virologic markers that would allow prospective detection of such individuals. In our family-based study, four of the five index cases implicated in the transmission of resistant virus were young children (2 to 5 years old) (13). In the household setting, avoiding treatment of ill index cases, specifically young children (13), would appear to reduce the likelihood of failure of drug prophylaxis as a result of the apparent transmission of resistant virus (7, 10, 11, 13). Despite the recovery of resistant virus from one-third Qf the rimantadine group, rimantadine-treated patients, the majority of whom were children or teenagers, had significant reductions in duration of fever and illness severity compared with those in the placebo-treated subjects. These findings are similar to those of an earlier study of H3N2 subtype illness (12) but are in contrast to those of another study in children with predominately HlNl subtype illness (20), both of which compared the therapeutic efficacy of rimantadine with that of acetaminophen. In the earlier H3N2 subtype study (12), illness severity scores later in therapy tended to be higher in rimantadine-treated children who shed resistant virus than they were in those who did not, but the differences were felt to be too small to be clinically detectable. In our familybased study, the subgroup of rimantadine-treated patients who shed resistant virus appeared to have somewhat slower illness resolution than did those from whom resistant virus was not recoverable. However, no late increases in symptom severity were observed in the overall rimantadine group or in the subgroup which shed resistant virus. A causal relationship between the recovery of resistant virus and prolongation of illness has not been established. Recovery of resistant virus may be more likely in patients who have more prolonged or quantitatively greater viral shedding for other reasons. In an avian model of influenza, therapeutic administration of amantadine beginning up to 3 days after infection resulted in reduced mortality compared with that after no treatment, despite the consistent recovery of drug-resistant virus by day 3 of treatment (1, 2). In the family-based study, the duration of fever and illness in rimantadine recipients who shed resistant virus tended to be less than that in placebo recipients. No evidence to date suggests that illness related to drug-resistant virus is more severe than that caused by drug-susceptible virus. Thus, rimantadine retains a net therapeutic benefit with respect to ANTIMICROB. AGENTS CHEMOTHER. symptom relief and functional recovery, despite the recovery of resistant virus from a portion of treated patients. Future studies are needed to assess the possible impact of resistant viruses through broader epidemiologic surveillance incorporating susceptibility testing, to ascertain the risk factors for emergence of resistant viruses, and to test strategies for reducing the selection and/or transmission of resistant viruses, such as short-course treatment (12), treatment with alternative antiviral agents or combinations of antiviral agents, and the combined use of chemoprophylaxis and immunization (22). ACKNOWLEDGMENTS We thank the participants and the many pursing and technical staff members who helped conduct this study. MariQn Oakes and Whaijen Soo, Hoffman-LaRoche, Nutley, N.J., assisted in the design and analysis of these trials. Maurice Harmop, Influenza Branch, Centers for Disease Control, provided the sequence data for the resistant isolates in the adult treatment study. These studies were supported by grants from Hoffman-LaRoche, the Veterans Administration, the National Institute of Allergy and Infectious Diseases, and by the University of Virginia General Clinical Research Center (RR ). REFERENCES 1. Bean, W. J., S. C. Threlkeld, and R. G. Webster Biologic potential of amantadine-resistant influenza A virus in an avian model. J. Infect. Dis. 159: Beard, C. W., M. Brugh, and R. G. Webster Emergence of amantadine-resistant H5N2 avian influenza virus during a simulated layer flock treatment progrqni. Avian Dis. 31: Belshe, R. B., B. Burk, F. Newman, R. L. Cerruti, and I. S. Sim Resistance of influenza A virus to amantadine and rimantadide: results of one decade of surveillance. J. Infect. Dis. 159: Belshe, R. B., M. Hall-Smith, C. B. Hall, and R. Betts Genetic basis of resistance to rimantadine emerging during treatment of influenza virus infection. J. Virol. 62: Betts, R. F., F. G. Hayden, and A. 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