Naturally Acquired Immunity to Pneumonia Due to Mycoplasma pneumoniae

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1 THE JOURNAL OF INFECTIOUS DISEASES. VOL. 147, NO.6. JUNE by The University f Chicag. All rights reserved /83/ $00.75 Naturally Acquired Immunity t Pneumnia Due t Mycplasma pneumniae Hjrdis M. Fy, Gerge E. Kenny, Marin K. Cney, Inez D. Allan, and Gerald van Belle Frm the Departments fepidemilgy, Pathbilgy, and Bistatistics, Schl f Public Health and Cmmunity Medicine, University f Washingtn, Seattle, Washingtn The immunity t Mycplasma pneumniae was studied by peridic cllectin f sera up t 15 years after infectin. Sera were tested fr antilipid antibdies by cmplement fixatin. Antibdy levels remained elevated fr tw t nine years after pneumnia but usually fell sharply after the secnd year in persns with milder symptms. Infectin rates were at least six times higher in cmparisn grups than in previus pneumnia patients. Schlchildren with serlgic evidence f infectin, but withut pneumnia, during the epidemic were nt prtected during the next epidemic (1974). Children with evidence f infectin during the first tw years f life were at n higher risk f clinical pneumnia (immunpathlgical respnse) at schl age than thse withut previus knwn expsure. The current study suggests that naturally acquired infectin induces partial immunity which lasts lnger after pneumnia than after mild infectins. Mycplasma pneumniae is a cause f primary atypical pneumnia, but many infectins are asymptmatic r cause nly mild symptms, such as pharyngitis and brnchitis [1, 2]. Epidemics ccur at intervals f fur t seven years, and reinfectin has been bserved [3-6]. Fr mre than 11 years we studied the incidence f pneumnia in a health.maintenance rganizatin, the Grup Health Cperative, in Seattle. Schlchildren experienced the highest attack rate f pneumnia due t M. pneumniae, and the rates declined with age, except fr higher attack rates amng persns wh were years f age, the general age f the parents f schlchildren [5]. Military recruit ppulatins have attack rates higher than schlchildren [7], a circumstance attributed t crwding and herding. In 1970 we encuntered a first case f repeated M. pneumniae pneumnia which kindled ur interest in the duratin f immunity fr this disease. Eventually five cases frepeated M. pneumniae pneumnia were bserved in members f the Grup Health Cperative [6]. Received fr publicatin June 28, 1982, and in revised frm February 3, This wrk was supprted by grants n. AI-I0695 and AI and cntract n. NOI-AI frm the Natinal Institute f Allergy and Infectius Diseases. Please address requests fr reprints t Dr. Hjrdis M. Fy, Department f Epidemilgy (SC-36), Schl f Public Health and Cmmunity Medicine, University f Washingtn, Seattle, Washingtn The fact that the attack rate f M. pneumniae appears lwer in children yunger than five years f age than in lder children has led t the hypthesis that primary infectin may sensitize the yung child s that mre severe manifestatins (enhancement) are experienced n expsure t this rganism a secnd time [2, 8, 9]. We have studied the duratin and extent f immunity and the frequency f reinfectin by lngterm bservatins f several ppulatin chrts in Seattle. A special effrt was made t study children wh had been infected with M. pneumniae early in life t see if such infectin predispsed t mre serius disease upn reinfectin. M. pneumniae epidemics ccurred in Seattle in and in 1974, prviding pprtunities t search fr reinfectins [5]. Althugh repeated infectins ccasinally ccurred in thse with previus M. pneumniae pneumnia, the frequency f repeated infectins was lwer than in the age-matched cmparisn grups. Subjects and Methds Ppulatins. The ppulatin grups under bservatin include thse with a histry f M. pneumniae pneumnia and, fr cmparisn, chrts f schlchildren and adults enrlled in prspective studies f acute respiratry diseases and families wh were investigated when ne member served as an index case fm. pneumniae 967

2 968 Fy et al. pneumnia and wh were subsequently studied lngitudinally. Patients with previus pneumnia. Pneumnia due t M. pneumniae was mnitred amng the 180,000 (average) members f the Grup Health Cperative f Puget Sund, Seattle, between 1963 and 1975 [5, 10]. The disease was diagnsed by islatin f M. pneumniae and/r significant increases in titer f CF (antilipid) antibdy in paired sera [11, 12]. A ttal f 947 cases f M. pneumniae pneumnia were recgnized by islatin f the rganism, and these patients were enrlled in the present fllw-up study. Beginning in 1967, these patients were sent a letter requesting infrmatin regarding pneumnia and influenza (minr respiratry diseases were ignred) every 18 mnths and were asked t dnate a bld specimen fr serlgic studies. The 18-mnth perid was a cmprmise; mre frequent cntact was impssible because f limited resurces. The physicians at the Grup Health Cperative were ntified by letter f ur attempt t demnstrate repeated infectins, and the recrds f the patients wh had had M. pneumniae pneumnia were marked t alert the physicians. Serum cllectin was carried ut in the hme if cnsenting patients still lived in the greater Seattle area but culd nt cme t the clinics. Specimens were smetimes shipped t Seattle frm participants wh had mved ut f state. Patients were als asked t call ur surveillance nurse immediately if they had a secnd attack f pneumnia r an illness that appeared t be similar t their previus M. pneumniae pneumnia. In such cases, thrat cultures and serum were cllected in the hme if nt cllected at the Grup Health clinic. Ntificatins f secnd attacks r pneumnia-like illnesses were rare. Cmparisn grups. Studies in schls. The incidences f infectin in tw chrts f basically healthy children and adults were measured by tests fannual samplings f sera. The first chrt cnsisted f nearly 400 elementary-schl children enrlled in 1966 in a prspective study fr determinatin f their rate f infectin with M. pneumniae [10]. The secnd chrt riginally cnsisted f 666 children five t 15 years f age enrlled in 1968 in a lngitudinal study f the efficacy f influenza vaccines [13]. Frm these tw chrts, bth sera and infrmatin n pneumnia and influenza were sught n an annual basis. A few children wh had M. pneumniae pneumnia when first infected were excluded in the subsequent analysis. Virus- Watch families. Sixty-fur families with newbrn infants were enrlled frm the Grup Health Cperative membership between 1965 and 1967 and studied fr tw years with intensive surveillance fr viral infectin, the Virus Watch [14]. Sera were cllected twice a year frm all family members except infants, frm whm n sample was taken until the age f 18 mnths. The Virus Watch study was terminated in September Beginning in 1972, sera were slicited frm these families n an annual basis fr fllw-up purpses. Families with an.index case fm. pneumniae. A study f infectins in 36 families that belnged t the Grup Health Cperative and that included a patient serving as an index case f M. pneumniae pneumnia was carried ut in [1]. The 59 secndarily infected family members, mst f whm were yung children, had varius, usually milder symptms (brnchitis, pharyngitis, r n symptms at all) and were mnitred in a fashin similar t that fr the pneumnia patients. Nt all individuals in these fur study grups cntributed a cmplete set f sera fr varius reasns. Participatin tapered ff as children left hme fr cllege r military service, divrces brke up families, and many individuals mved ut f the area. Labratry methds. The labratry methds fr islatin (slid and diphasic media) and the serlgy f M. pneumniae infectins have been described previusly [11, 12]. The lwest dilutin f serum tested was 1:4. Because the sera were cllected ver many years, sera were nt always tested in the same test run. Hwever, in cases f apparent furfld r greater increases in antibdy titers, the relevant sera were retested tgether fr cnfirmatin f infectin r reinfectin. Statistical methds. Infectin rates per 100 persn-years f bservatin were calculated. Ninety-five percent cnfidence intervals were calculated assuming that the number f infected persns in a particular chrt and age grup fllwed a Pissn distributin. The cnfidence limits n the Pissn parameter were then cnverted t rates per 100 persn-years. Fr infectin rates during the M. pneumniae epidemic, cmparisns amng

3 M. pneumniae Pneumnia Observed Predicted -- Pneumnia \ Nn-pneumnia " \ \. \ '0\ \ \,, \ \0,,,, , Time in years Figure 1. Decay f CF antibdies t Mycplasma pneumniaein patients five t 14years f age with pneumnia and thse with milder infectins. The slpes f the calculated regressin lines shw that antibdies decayed faster in persns withut pneumnia (P < 0.01). See Subjects and Methds fr a descriptin f the methd f fitting f regressin lines. subgrups were dne by means f standard 2 x 2 X 2 tests. Linear lgistic mdels were fitted t the percentage f persns having antibdies as a functin f time [15] (figures 1 and 2). Specifically, a weighted linear regressin f lge [P/(l-p)] vs. time was fitted, where p = prprtin infected; the weights were estimated frm n/[p(l-p)], where n is the number f subjects at risk in ne f the grups at a particular time, and p is as defined befre. The cmputer prgram GLIM was used t calculate the regressin lines [16]. Results Illness episdes investigated amng previus M. pneumniae patients. Amng the mre than 900 patients with previus M. pneumniae pneumnia, 34 episdes f repeated pneumnitis r febrile brnchitis were reprted and investigated at the time f subsequent illness. In five f the 34 cases, a repeated ccurrence f M. pneumniae pneumnia was demnstrated by chest rentgengram, islatin f the agent, and increase in antibdy titer [6]. The intervals between repeated episdes f pneumnia varied between three and 10 years. These illnesses were relatively mild and uncmplicated and have been described in detail previusly [6]. A sixth persn had a significant increase in antibdy titer between the sampling f acute- and cnvalescent-phase sera but had a negative thrat culture at the secnd episde f an illness. He was 33 years ld in 1965 at the time f riginal pneumnitis and 40 years ld at the time f the secnd illness. He cmplained f a febrile cugh reminiscent f the first illness (n chest rentgengram was taken). His sn als had M. pneumniae pneumnia in 1965 at the age f five years, but he had n illness despite a furfld increase in antibdy titer at the time f his father's secnd illness. The remaining 29 illnesses were nt due t M. pneumniae. Serlgic fllw-up fpatients with M. pneumniae pneumnia. A ttal f 471 patients with M. pneumniae pneumnia cntributed at least tw serum samples (mean, 3.2; range, tw t eight per persn) in the years fllwing their episdes f pneumnia. Significant increases in levels f antibdy indicative f reinfectins ccurred in nly 24 instances (twice in ne persn). Mst increases in antibdy titer were bserved in persns with preceding reciprcal titers f ~8, but tw subjects had increases frm 1:32 t 1:128 (in I8-mnth intervals). Of the 23 persns with serlgically recgnized reinfectin, nly three reprted pneumnia-like illnesses fr the interval between serum samples. One was a pneumnia patient described previusly [6]. A secnd was a tw-year-ld girl wh had M. pneumniae pneumnia in 1973 and wh was diagnsed as having pneumnia again in The third was a by wh had M. pneumniae pneumnia in 1969at 13 years f age, and he is the nly persn with tw reinfectins. He had significant increases in antibdy titers between 100 -s 80 Cl ~ 60 lj.: (j ~AO ~ '! _~. t:: Observed Predicted ~ 2a -- Pneumnia <5 yld ~ Pneumnia 20-49yld O+---~--,-----r r----r-----r-----, Time in years Figure 2. Decay f CF antibdies t Mycplasma pneumniae in patients yunger than five years f age and patients years f age. The slpes fthe calculated regressin lines shw that antibdies decayed faster in yung children than in adults (P < 0.05). See Subjects and Methds fr a descriptin f the methd f fitting regressin lines.

4 970 Fy et al. Table 1. Infectin rates per 100 persn-years f bservatin by age at entrance int bservatin in persns with and withut previusly knwn Mycplasma pneumniae pneumnia. Age in years Grup Ttal Previus M. pneumniae pneumnia 0.7 (146) 1.0 (680) 1.2 (565) 0.8 (130) 1.1 (90) 1.5 (204) 1.7 (119) 4.7 (43) 1.2 (1,977) [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] Cmparisn 1966 chrt schlchildren 11.2 (758) 9.2 (130) 10.9 (888) [ ] [ ] [ ] 1968 chrt schlchildren 8.8 (832) 8.0 (346) 8.6 (1,178) [ ] [ ] [ ] Virus Watch 6.6 (426) 6.0 (134) 9.6 (31) 4.8 (21) 5.2 (329) 6.2 (273) 4.0 (101) 0(4) 5.9 (1,319) [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] NOTE. Data are infectin rates per 100 persn-years (n. f persn-years bserved) [95010 cnfidence limits n infectin rate per 100persn-years]. Fr a descriptin f subject grups and methd used t calculate cnfidence limits, seesubjects and Methds. serum samples taken in 1970 and 1972 and between 1975 and 1977, and he had been diagnsed as having pneumnitis f the left lwer lbe in Infectin rates amng previus M. pneumniae pneumnia patients and cmparisn grups. The rate f significant antibdy titer increases was significantly lwer amng previus M. pneumniae pneumnia patients than amng cmparisn grups in the years f fllw-up (table 1). A straightfrward calculatin f infectin rates per year was pssible nly fr the cmparisn grups frm which serum samples were taken annually. Antibdy levels f pneumnia patients ften stayed elevated fr three years, precluding detectin f pssible reinfectin during that time. Calculatin f reinfectins was thus started three years after the episde f pneumnia. Since the true interval between the drawing f samples was 18 mnths fr the previus pneumnia patients, and the lnger interval culd have precluded detectin f an increase and decrease in antibdy titer, we calculated reinfectin rates as if the interval between samples was 12 mnths. This handling f the data is biased twards verestimating annual reinfectin rates amng previus pneumnia patients. Regardless, the infectin rates are abut sixfld higher fr the cmparisn grups than fr the previus pneumnia patients under the age f 15 years; with increasing age f the subjects, the difference becmes smaller. Infectin rates during the 1974 epidemic. The epidemic f 1974 ffered anther pprtunity t cmpare infectin rates amng previus M. pneumniae pneumnia patients and the cmparisn grups by age (table 2). The pneumnia patients wh were included had serum samples drawn that spanned the epidemic peak perid (May-Octber 1974). The infectin rate amng the cntrl grups is mre than 10-fld that f the previus pneumnia patients. Because samples were drawn frm the pneumnia patients nly every 18 mnths, the likelihd f detecting a reinfectin may have been less than in the cmparisn grups. Samples were drawn frm schlchildren and families each spring, and an increase in antibdy titer that was recrded in the spring f either 1974 r 1975 was included. High infectin rates ( ) were seen in the five- t nine-year-ld schlchildren f bth the 1966 and 1968 study chrts. There was n difference in infectin rates between thse children with and withut previus serlgic evidence f infectin. Reprts f pneumnia amng the schlchildren were infrequent, and underreprting may have ccurred as a result f incmplete recall. Only ne fthe 16 schlchildren reinfected (table 2) reprted pneumnia in assciatin with the reinfectin episde. One child had serlgic evidence f three infectins, nne f which was assciated with pneumnia. Based n lack f previus antibdy, 40 children appeared t have had a primary infectin in 1974: tw children

5 M. pneumniae Pneumnia 971 Table 2. Infectin rates in percentage by age (at entrance int bservatin) during the 1974Mycplasma pneumniae epidemic amng persns with a histry f M. pneumniae pneumnia and amng cmparisn grups. Age in years Grup Previus M. pneumniae pneumnia 0(14)* 2 (53)t Cmparisn 1966 chrt schlchildren Previus furfld increase 63 (19) N previus furfld increase 58 (40) 1968 chrt schlchildren Previus furfld increase 40 (10) N previus furfld increase 47 (57) Virus Watch 24 (66) 27 (15) NOTE (31)* 0(3) 30 (20) 63 (8) (6) 0(2) (1) 10 (50) (11) 12 (34) (12) Data are infectin rates in percentages (number f persns bserved). Fr the tw grups f schlchildren, infectin rates are shwn fr thse wh had had significant (furfld r greater) increases in titer f antibdy befre the epidemic and fr thse wh had nt. * Difference frm cmparisn grups f the same age was significant (P < 0.05). t Difference frm cmparisn grups f the same age was significant (P < 0.01). (teenagers at the time) reprted pneumnia in assciatin with their first dcumented furfld increases in antibdy titer. Decay f antibdy. Decay f antibdy in pneumnia patients five t 14 years f age at the time f illness and schlchildren f the same age having antibdies in lngitudinal studies is shwn in figure 1. Mst f the nnpneumnia grup acquired their antibdies during the M. pneumniae epidemic. Fr bth grups, persns with evidence f repeated infectin shwn by furfld r greater increases in antibdy levels were excluded. Initially, persns with M. pneumniae infectins withut pneumnia had antibdy titers slightly higher than the cnvalescent-phase antibdy titers f M. pneumniae pneumnia patients. Whereas mst pneumnia patients maintained sme antibdies fr several years, antibdy levels declined faster in thse with milder infectins. The slpes f the calculated regressin lines differed significantly (P < 0.01). Similarly, antibdy decay in adult pneumnia patients vs. patients less than five years f age was.cmpared (figure 2). The decay f antibdy was significantly slwer in the lder age grup than amng yung children (P < 0.05). Search fr infectins in infants studied frm birth. A grup f special interest is 25 infants first bserved fr a minimum f tw years in the Virus Watch surveillance ( ), and then studied frm 1972 with annual tests f sera. Nine f the 25 infants had antibdy t M. pneumniae in the first tw years f life, and their infectin 8 (12) rates were insignificantly lwer (6070 in 35 persnyears f fllw-up) than thse in infants withut evidence f infectin in the first tw years f life (9070 in 55 persn-years f fllw-up). Only ne child reprted pneumnia when she was f schl age. She had had n serlgic evidence f infectin until 1978, when at 12 years f age she had an eightfld increase in antibdy titer (frm n detectable titer). An additinal nine children participating in the Virus Watch studies had antibdy t M. pneumniae in their first sample serum cllected at 18 mnths f age (in the late 1960s) but had n serlgic fllw-up. Our Grup Health pneumnia files were searched, and available families were asked abut subsequent pneumnia. Nne f the children had been diagnsed as having had M. pneumniae pneumnia. Fllw-up f family members expsed t an index casefm. pneumniae pneumnia in Amng the 59 family members identified as infected withut pneumnia when expsed t an index case f pneumnia in , tw children, wh were six and 15 years f age at the time f asymptmatic infectin, reprted that they later had pneumnia. Neither had had diagnstic tests fr their pneumnia. One acquired his pneumnia during military service. Ten children were yunger than five years f age at the time f infectin. Nne reprted pneumnia in subsequent years. One family (clsely acquainted with the authr) that included fur children yunger than six years f age was studied until The

6 972 Fy et al. children had nly minr symptms in 1965 when M. pneumniae infectin was intrduced by the mthers. In spite f repeated cllectins f serum and thrat specimens during febrile respiratry illnesses frm 1965 t 1980, n evidence f repeated infectins was fund. Discussin The mst striking finding in this analysis is that M. pneumniae pneumnia cnferred substantially mre prtectin against serlgically recgnized reinfectin than did infectin that caused milder symptms and that antibdy decayed mre slwly in pneumnia patients than in thse wh had milder symptms. Crder et al. [17] were the first t bserve that antibdy lasted lnger after pneumnia than after mild infectin. Reinfectins, which were demnstrated serlgically, were assciated with a diagnsis f pneumnia in nly three f 23 patients with previus cases f pneumnia, a prprtin similar t the rate generally bserved in lder schlchildren, % [10, 18, 19]. A similarity t the current findings may exist in the hamster mdel [20]. Animals previusly infected intranasally with either a virulent r avirulent M. pneumniae strain were prtected frm pneumnia when challenged with a virulent strain, whereas animals vaccinated parenterally with an avirulent strain were nt prtected despite high levels f serum antibdy. This suggests that serlgic antibdy alne is nt prtective but that a lcal immune mechanism, pssibly secretry antibdy, may be imprtant in prtectin; thus the mre severe natural infectins result in prtectin n rechallenge. Since diagnsis was accmplished in the present study by serlgical means fr mst reinfectins, the specificity f the test is imprtant. Leinikki et al. [21] have demnstrated that patients with pancreatitis shw furfld increases in levels f antibdy t the lipid antigen f M. pneumniae withut any evidence f M. pneumniae infectin. Mrever, the antibdy detected was mstly IgM. Pnka et al. [22] have shwn that f hspitalized patients wh had furfld increases in antibdy levels did nt have pulmnary manifestatins f infectin. It is nt presently pssible t evade crss-reactins with the lipid antigen because all available tests (CF, metablic inhibitin, and mycplasmacidal assays) measure antibdies t the lipid antigen [23]. In the present study, specificity was suggested in tw ways. (1) Fewer previus M. pneumniae patients shwed furfld increases in titer than patients wh were nt knwn t have been previusly infected, whereas an anamnestic recall wuld be expected t ccur mre frequently in the previus pneumnia grup, and (2) peak numbers f sercnversins were bserved during epidemic perids, particularly amng the schl chrts and Virus Watch families [5]. Thus, mst f the antibdy increases detected wuld appear t be specific. A mre sensitive antigen may detect mre antibdy increase but may als increase the risk f false-psitive results [8, 23]. It has been pstulated that an infectin with M. pneumniae in infancy may enhance the immune respnse s that subsequent infectin becmes mre severe, resulting in pneumnia [2, 8, 9]. The fllw-up f nine children infected in infancy and cmpared with infants nt s infected did nt indicate any increased risk f M. pneumniae pneumnia. And yung children infected with M. pneumniae in the early 1960s did nt have subsequent serius disease. Rather, the data suggest that at least partial prtectin is acquired frm infectin early in life. In military ppulatins preexisting antibdies crrelated with prtectin [24], and the killed M. pneumniae vaccines were abut effective against pneumnia [25, 26]. Hwever, vaccine did nt prtect frm clnizatin and shedding. We believethat the current study is sufficient t suggest that acquired immunity explains the age distributin f M. pneumniae pneumnia (highest rate in schlchildren) bserved in civilian ppulatins. This age distributin is reminiscent f sme ther childhd diseases, such as mumps and rubella. It is likely that the decreasing attack rates after the age f 10 years represent acquired immunity, althugh mre than ne episde f infectin may be necessary t build up resistance. The study suggests that immunity wanes with time and is prbably reinfrced by subclinical infectins. References 1. Fy, H. M., Graystn, J. T., Kenny, G. E., Alexander, E. R., McMahan, R. Epidemilgy f Mycplasma pneumniae infectin in families. J.A.M.A. 197: , 1966.

7 M. pneumniae Pneumnia Fernald, G. W., Cllier, A. M., Clyde, W. A., Jr. Respiratry infectins due t Mycplasma pneumniae in infants and children. Pediatrics 55: , Lind, K., Bentzn, M. W. Epidemics f Mycplasma pneumniae infectin in Denmark frm 1958 t Int. J. Epidemi!. 5: , Nah, N. D. Epidemilgy f Mycplasma pneumniae infectin in the United Kingdm: an analysis f reprts t the Public Health Labratry Servicef England and Wales. Infectin 4(Supp!. 1):25-28, Fy, H. M., Kenny, G. E., Cney, M. K., Allan, I. D. Lng-term epidemilgy f infectins with Mycplasma pneumniae. J. Infect. Dis. 139: , Fy, H. M., Kenny, G. E., Sefi, R., Ochs, H. D., Allan, I. D. Secnd attacks f pneumnia due t Mycplasma pneumniae. J. Infect. Dis. 135: , Steinberg, P., White, R. J., Fuld, S. L., Gutekunst, R. R., Chanck, R. M., Senterfit, L. B. Eclgy f Mycplasma pneumniae infectins in Marine recruits at Parris Island, Suth Carlina. Am. J. Epidemi!. 89: 62-73, Brunner, H., James, W. D., Hrswd, R. L., Chanck, R. M. Measurement f Mycplasma pneumniae mycplasmacidal antibdy in human serum. J. Immun!' 108: , Smith, C. B., Friedewald, W. T., Chanck, R. M. Inactivated Mycplasma pneumniae vaccine: evaluatin in vlunteers. J.A.M.A. 199: , Fy, H. M., Kenny, G. E., McMahan, R., Kaiser, G., Graystn, J. T. Mycplasma pneumniae in the cmmunity. Am. J. Epidemil. 93:55-67, Kenny, G. E., Graystn, J. T. Eatn pleurpneumnialike rganism (Mycplasma pneumniae) cmplement fixing antigen: extractin with rganic slvents. J. Immun!. 95:19-25, Kenny, G. E. Mycplasmata. In E. H. Lennette, A. Balws, W. J. Hausler, Jr., J. P. Truant [ed.]. Manual f clinical micrbilgy. 3rd ed. American Sciety fr Micrbilgy, Washingtn, D.C., 1980, p Fy, H. M., Cney, M. K., Allan, I. Lngitudinal studies f types A and B influenza amng Seattle schlchildren and families, J. Infect. Dis. 134: , Cney, M. K., Fx, J. P., Hall, C. E. The Seattle Virus Watch. VI. Observatins f infectins with an illness due t parainfluenza, mumps and respiratry syncytical viruses and Mycplasma pneumniae. Am. J. Epidemi!. 101: , Andersn, S., Auguier, A., Hauck, W. W., Oakes, D., Vandaele, W., Weisberg, H. I. Statistical methds fr cmparative studies: techniques fr bias reductin. Wiley, New Yrk, 1980, p Baker, R. J., Nelder, J. A. The GUM system. Numerical Algrithms Grup, Oxfrd, 1978, p Crder, L., Cuadrad, R., Hall, C. B., Hrstmann, D. M. Primary atypical pneumnia: an epidemic caused by Mycplasmapneumniae. J. Pediatr. 71:1-12, Dwdle, W. R., Stewart, J. A., Heyward, J. T., Rbinsn, R. Q. Mycplasma pneumniae infectins in a children's ppulatin: a five-year study. Am. J. Epidemi!. 85: , Saliba, G. S., Glezen, W. P., Chin, T. D. Y. Mycplasma pneumniae infectin in a resident bys' hme. Am. J. Epidemi!. 86: , Fernald, G. W. Immunlgic aspects f experimental Mycplasma pneumniae infectin. J. Infect. Dis. 119: , Leinikki, P.O., Panzar, P., Tykka, H. Immunglbulin M antibdy respnse against Mycplasma pneumniae lipid antigen in patients with acute pancreatitis. J. Clin. Micrbi!. 8: , Pnka, A., Pnka, T., Sarna, S., Penttinen, K. Questinable specificityf lipid antigen in the Mycplasma pneumniae cmplement fixatin test in patients with extrapulmnary manifestatins. Jurnal f Infectin 3: , Kenny, G. E. Serlgy f mycplasmic infectins. In N. R. Rse and H. Friedman [ed.]. Manual f clinical immunlgy. 2nd ed. American Sciety fr Micrbilgy, Washingtn, D.C., 1980, p McCrmick, D. P., Wenzel, R. P., Senterfit, L. B., Beam, W. E., Jr. Relatinship f pre-existing antibdy t subsequent infectin by Mycplasma pneumniae in adults. Infect. Immun. 9:53-59, Mgabgab, W. J. Prtective efficacy f killed Mycplasma pneumniae vaccine measured in large-scale studies in a military ppulatin. Am. Rev. Respir. Dis. 108: , Wenzel, R. P., Craven, R. B., Davies, J. A., Hendley, J. 0., Hamry, B. H. Gwaltney, J. M., Jr. Prtective efficacy f an inactivated Mycplasma pneumniae vaccine. J. Infect. Dis. 136(Supp!.):S204-S207, 1977.