Live, orally given poliovirus vaccine. Effects of rapid mass immunization on population under conditions of massive enteric infection with other viruses. 1960.

Public Health Classics THE JOURNAT O F T H E American Medical Association PublisheilUtder the Auspices of the Booril of Trustees vol". 173, NO. 14 CHICAGO, ILLTNOISCorvrrcgr, 1960, rv Axrnrcrx Meprcrr, AssocBrroN AUGUST 6, I9OO LIVE, ORALLY GIVEN POLIOVIRUS VACCINE EFTECTS OF BAPID MASS IMMUNIZATION ON POPULATION UNDER CONDITIONS OF MASSIVE ENTERIC INFECTION WTTH OTHER VINUSES Albert B. Sabin" M,D" Cincinnati, Manuel Ramos-Alvarez, M.D,, Josd Alvarez-Amezquita, M.D., Mexico, D. F, William Pelon, Ph.D., Riehard H, Michaels, M.D, Ilya Spigland, M.D., Meinrad A. Koclr, M.D., ]oan M. Barnes, Ph.D. and lohng S. Rhim, M.D., Cincinnati E N THE absence of enteric infection u'ith I I other viruses, the feeding of a single type f I of live poliovirus vaccine to children who - have not previously been infected with the same type of poliovirus usually results in viral multiplication in the intestinal tract, antibody for- mation, and ccmplete or partial resistance to rein- fection. The use of live poliovirus vaccine in areas with climatic and hygienic conditions which permit extensive dissemination of naturally occurring polio- viruses and other enteric viruses througbout the year has been eomplicated by the problern of viral interference,' rvhich we have attempted to over- come by the tactics to be deseribed in this com- munication. Tlre stuclies of one of us (lvl.R.-A.) and associates * in Mexicrr and of Plotkin and Koprowski " in the Belgian Congo have shown that vaccination pro- grams in large subtropical or tropical cities tlrat are spread out over a period of many weeks and inelude only a small fraction of the suscePtible population not only fail to immunize a considerable proportion of the vaccinated children but also have little or no infuence on the dissemination of the ever- preser:t, uaturally occurring paralytogenic strains of grliovirus. Aecordingly, it seemed nec€ssary to detirmine whether at least temporary dominanee over the naturally occurri:ng lrclioviruses and otber Ihe pbenomenon of virol interference musl be token info occounl in planning lhe use of five poliovirus voccine in oreos where conditions fovor the exlensive disseminolion o{ notvrally occuring polioviruses. Experi- ence wifh feeding a lrivalent voccine fo 26,033 children in o tropicol city of IOQ,- 00A populotion led to the conclusion lhol interference wos overcome by moss feeding of yoccine to 86'h of, oll children under I I yeors within o period of obouf four doys, and that, becouse disseminofion of rhe polio- virus wos self-lirnifed, q second feeding of hivofenl voccine wos necestory to achieve lmmunization of olrnosf all children. Recom- mendofions ore here f ormvlaled for the erodicolion o( poliomyelitis, but they opply only to subfropicol and tropical regions with exfensive disseminolion of vorious enleric viruses ond nol fo temperdte zones wifh good sonifolion ond hygiene during cerloin periods of fhe ;,eor ond under condilions of low or obsenl disseminolion of enferic viruses. Fm tbe Clrildrm'g Hosptte! B-esarch Foundation, Univusity of Cincinnati Gollege of Mediciae; Hospital Infantil, Mexico, D. F.; ald thc Ministry of Health, Mexim.: ini$try nf fteetth. xlm. Rertl in pert beforr the 73rd aronual necting of ttre Association of American Physicians, Atlantie City, May 3, 1960. 195 1s2l Bulletin of the World Healfr Organization, 1999,71 (2) L5.22 POLIOVIRUS VACCINE-SABIN ET AL 4-. - - J.A.M.A., Aug. 4 1960 Metbods of Study ad Results This study was carried out in Toluca, Mexico, in August, 1959. One of us (M.R.-A.) and his associates in Mexico were responsible for the ffeld work and the collection of speeironens, and all the virologieal and serologic work was carried out in Cincinnati. Toluca, loeated at a latihrde of about 19 degrees north, has a total populadon of about 100,000. A serolo$c suryey of the population iust before initia- tion of tlre vaccine program indicated that 90 to 100% of the chil&en become immune to all three types of poliovirus during the Srst 4 years of life (ffS. I).According to available data for the past ffve TesLE l.-Proportion of ChWren of Ind.bated Age Fed Sirgle Dose of TtioabntVaccbte ir.Toluca, Merico, August, 1959 PersoDs of fed PelloEr ot IDalicrteal AgG, VscciD!, Itrdissted AgG Agc, Yr. No, No. Vcccbrted, % 1 . . . . . . . . . . . . . . . . . . . , . . , . 2 , t 0 3 z . W I A 2 . . . . . . . . , . . . . , . , . . . . . . , . S , l I 5 7 2 , 3 6 1 7 1 8 . . . . . . . . . . . . . . . . , . . . . . . . 8 . 3 1 0 2 . 4 5 8 7 1 { . . . . . . . . . . . . . , . , . . . . . . . . 8 , 2 9 t 1 , 5 7 5 a 5 . . . . . . . . . . . , . . . . . . . , . . . . 9 . 8 8 6 2 , 4 1 0 t 3 6 - 1 0 . . , . . . . . . . . , . . . . . . . . . 1 1 , 7 0 E 1 0 , 9 { 2 9 3 < 1 . r 0 . . . . , . . . . . . 8 0 . 4 ? 6 2 6 , 0 3 g t 6 Total populstiou, s l l a ies . . . . . . . . , . . . . . . 108 ,072 26 ,088 years, this eity has been paying an average price of 14 paralytic cErses per year for this naturally ac' quired immunity. Twenty cases of paralytic polio- myelitis had already been reported in 1959, when our study began during the second week of Au- gust. Rectal swabbings from I,892 persons-children urterfering'enterie viruses eould be achieved by feeding a mixture of all thnee types of poliovirus vaceine to almost all suseeptible chil&en within a period of a few days. The reason for using'a triva- ient mixture irutead of feeding the three types t 2 3 4 5 6 - ' 0 9 0 80 70 90 ueo E70 a Oeo0- ' 3 or- z !J 40 o 30 r. !J ?o n / }_. . . TYPE I }.-"..o TYPE II O-.-{ TYPE ltr TNIPLE NEGATIVES t o { J 4! O-l- 7-l l MOS Fig. l.-Poliovirus antibodies in children in Toluca, Mexi. co, iust before feediag vsccine. Numbcrs above age groups indicatc nurnbcr of persons in eacb group tested. separately at intervals of four to six weela, which is iegarded as optimum during the cold months of the year in temperate zones, was to implant all three types of the vaceine strains in the largest number of children and then let nature help in their further dissemination in the community. qt '6 o o- c q,() (l) o- I TYPE I POUO EI TYPE U POLIO N TYPE III, POLIO trI NOT POLIO (NP) Fig. !.-Incklencc of various b?es of poliovinrs and other enteric viruses (NP) in childrcn from birth to l0 yeare of age and in their motbers, Toluce, August 1959, iust beforc feeding vrccinc. Nusrbcrc ebove csch column indieate number of per- sons in cach age group tcstcd- Bufletin ol the Wortd Health Organization, 1999,77 (2) 197 Vol. 173, No. 14 POLIOVIBUS VACCINE-SABIN ET AL 1523 among the motlrers of these young children was surprising-57o for poliovirubes and L67o for other enteric viruses. It was in this setting that a single dose of triva- lent poliovirus vaeeine (10 o'" to 10 "'" 50% tissue- culture dose [TCD66] of each type) was fed to a total of 26,&33 children under LI years of age-86% of all the children of this age group in Toluca. The actual proportion of children of each age group who received the vaccine is shown in table 1. Onlv four days were required to feed tlre vaecine to 97% of the children who reoeived iL To determine how efiectively the attenuated polioviruses competed with the other enteric vi- ruses and how long and extensively polioviruses continue to disseminate in such a community after tbis rapid and massive 4rtiffcial seeding over 3,139 rectal swabbings were obtained during the subse- quent three months and tested for virus. A total of aged from birth to 10 years and tbeir mot}ers-ob' tained at ratrdom in different parts of the eity within a few days before the vaccination program, were each tested in monkey kidney and human epi- thelioma (HEP 2) tissue cultures as well as in nev' born mice. The results, summarized in ffgure 2, T,tnr-a 2.-Self-Limiteil Characier of Dissemi:rctfum of Polio' alruses After Ropid Mass Oral Administraibn ol Trioabnl Yaccinc in a Tropicol CommunitY' B€fore VscciDe. 8 Uo. Aug., 1959 Attcr vf,cclDe ' f o td Do . l es t sd . , . . . . . , , . . 719 411 IleldeDcr of pollorinrses (P) ...,.,...... w.W 0.7% beialeDcr ol otber erteric viruses (FP) ,, il.8% fr.$% Brtio NP/P {.6 724 -'cr."i", ratr of Dolio14ruge6 aad otber erteric rinrgeE amoDg l- to 5.yrar-olal cbildreD sAectcd at rsDdo& betoF aDd I rbo, rJter siDglc dosc f!08.!.10E.3 TCDro) ot tdr'rlest orsl Doliovims veceiD. (Sabia) ted rrithi! s tev days to 80% ot chlldrc! (rcebom !o 10 yerB) lD Tolucu. llerico, Ca.ffler ratc is baseil ol debctlo! ol t4rus in si8gle rectal swotblDg- rctilal eauJer rrte is Bortrershrt blgher. 7 - f r M o N T H S ( 5 0 ) | Y E A R ( 4 7 ) PRE-NONE TRIPLE POSITIVE bJ t ' a o n F z ld{) E ld I lo0'1 e o 1 a o l 'ror 6 0 J uo { 401 301 ?3M o {at PRE I I I r00 90 80 70 60 50 40 30 ?o t o f--r. , at_tirol PoLro+ PoLto PRE - I9% TRIPLE POSITIVE / ts+1- -A. . /\ /\./ - ,Ax_ ONLY 4 6 8 t? loo 90 80 70 60 50 5 - l O Y r s . l n c l u s i v c ( 8 2 ) lO0 1 PRE- 86 % TRIPLE POSITIVE 90 80 70 60 30 20 t o o il J,rf ---/'^t\--'40 30 20 l o o PRE I 2 3 4 6 9 1 2 P R E t Z 3 4 6 8 Fig" 3.-Potioviruses and otber enteric viruses in gmup of 274 c,trildren tested iust before cnd at weeHy intervels after single dosc of trivalent orally given poliovirus vaccine, Toluca, August to November, 1959. Number in parenthesis next to each age grroup indicates number of pe,rsons tested. indicate that tbe special methods used revealed a mueh higher incidence of natural enteric viral in- fection than has ever been demonstrated before- beginning during the ftrst days after birth and reaching a peak of 72% during the lst year of life. Beginning with l-month-old children, polioviruses were isolated from every age group tested" with a peak rate of 14 to L5% in children between 7 months and 2 years of age. The high infection rate 2,711 viruses were recovered from the 5,031 pre- vaecination and postvaccination specimens, and the use of special methods permitted us to classify them rapidly as polioviruses or nonpolioviruses. The results, sbown in ffgure S for a group of 274 children who were tested repeatedly over a period of I months and in ffgure 4 for hundreds of dif- fereut children randomly chosen before and at monthly intervals after vaceination, indicate the fol- IOO l so' l 8 0 J . ,o1, 6 0 { s01 40 i 301 zo lll '3ffi PRE uJ. z F' n otr F. z td o E LJ 0- lo0 90 80 70 60 50 20 t 0 0 P R E t ? 3 4 6 8 t ? 198 Bulletin of the World Health Organization, '1999,77 (Z', Iowing conclusions. I' While tlie total incidence of enteric viruses varied little or not at all during the three months, the polioviruses became dominant TesLE 3.-Deoehpment of Poliooirus Neutralizine Anti- bodies During Comparabb Tm-Week Periodln Two Merican Citiest Toluc&- Af ter I Dose of Trivalctrt Qneretaro- VocciDe No VacelDe 'CoorcIsioD rBtes lor iDdicated type are t,used oD sirnultaDeous tesLs o! prirsd senlus trom origiDdly neg&tlve childreE. i Cbildren without sntil)ody vho were corrier€ of Dolioviru$ rt time first blood spctirncu Fes obtalned are excluded from this calcul&tioD. during the fust three rveeks after vaccination, the isolation rates being as high as 40 to 70Vo in the 1524 POLIOVIRUS VACCINE_SABIN ET AL. 96% 94% rooTo 85% 88% 88% poliomyelitis enteric viruses was about 5L% betorc and also three months after the vaccination pro- gram, while the isolation of polioviruses dropled from 1I to 0.7Vo, a sixteenfold diminution. The serologic effect of this temporary dominance of the polioviruses is shown in table 3, in which the antibody conversion rates during the first 10 weeks after the mass feeding of a single dose of trivalent vaccine in Toluca is eompared with the natural antibody conversion rates in the city of Queretaro, which is similar in size, climate, hygi- enic conditions, and geography, but in which no vaccine was given. The incidence of antibody con- version 10 weela after the single dose of trivalent vaccine in single-negative, double-negative, and triple-negative children is shown in table 4. The extent of interference resulting from the simultane- J.A.M.A., Aug. 6, 1960 @ Type I Potio ElType Il Polio WTypeEPal io Eruot porio (r,rp) 3 YRS. 4 YRS. 5 YRS. 977" 977. lOO7" 9l % too% 977" rOO% IOO% 877" rOO% 9t7o 94% tOO7. 96% 967o No. % -zlw 6 sl33 10 5/38 l8 No. l % 3ri/53 .t8 611i7 8E 3S/i6 l8 q) .: o o(L c(l)() (l, o- tYR. 2 ) Antibodyf I 57% 837. 75% % {II n% 83% 7e% Positive$I 43?o 777; 797. Fig.4.-Incidence of various lrees 9f poliovirus and other enteric viruses (NP) in l-to-$year-old children randomly se.lected just before and at indicated monthly intervats after single-dose of trivaleni oraily given poiionitor vaceine, 1.ol""", eugptt to November, 1959. Nurnbers above each eolumn refer to number of children tested d-uring-each period. different age goups. 2. The early dominance of the polioviruses was followed by a quick declinermore rapid in the children over 2 years of age-whicb by the end of three months reached a point that was markedly lower than before the mass feeding of the live polioviruses vaccine. The self"limited character of dissemination of polioviruses after this rapid mass feeding of live polioviruses vaccine, under conditions which favor the continued extensive dissemination of enteric agents, is strikingly brought out in table 2, which shows the results of tests on 1,135 randomly seleeted 1-to-5-yearold children. The isolation rate of non- pus administration of all three types and the domi- nance of the type 2 strain in this mixture are elearly evident. Thus, for type l, the conversion rate was 100% in the small number of single-ne gative, SEV* in the double-negative, and 55!" in the triple-nega- tive reaetors; for type 3 it wis 59, 42, and 37%, respectively; while for type 2 there was no di$er- ence in the three categories, the conversion rates being 82, 80, and 83%- An analysis of the data (table 5) on the children from whom rectal swabbings were obtained at weekly intervals indicated two signiffcant facts. l. Many children who developed antibody did not 50 30 20 ro o 47 t20 :iif;. ' . :. PRE t t z YRS. Buf letin of the Wor'ld Health Organization, 1999, TI {21 199 VoL 173, No. 14 excrete sumeient virus to be detected in the reetal swabs. 2. Some of the children who had no demon- strable antibody at 10 wee.ks excreted enough poho- virus to be detected in at least one of the rectal swabbings obtained during the ffrst 4 weelcs. When the children in the second category are ineluded. one obtains an estimated total infection rate af Wo for fire !, VTV, for type 2, and 47Io fot type 3 dur' ing ihe ftrst 10 weeks after the single trivalent dose of-vaccine. Further inalysis of thJse data indicates that among the total number infected, antibody was demonshable at 10 weeks in 86% for firc l, 947o for type 2, andg?Vo for type 3. POLIOVIRUS VACCINE-SABIN ET AL. 4 30 zo logic data shown in table 6, which indicate that among the children who failed to develop antibody durinf the ffrst 10 weeks very few converted dur' ing the subsequent 11 weeks. At this point the re' miiniog negative children in the group that were being followed serologieally were fed anotber dose of trivalent vaccine and they then exhibited a fiu- ther marked immunogenic effeet. The total conver- sion rate after the two doses of vaceine was 96% for types I and 2 end 1Wo for firc 3, despite the interfering effects resulting from the massive infec- tion with other enteric viruses (which was still 55% in January, 1960) and the simultaneous administra' 1525 (l, 'd I q) o E 90 80 70 60 TYPE T lowcckr Altff lst DOSE tO ALL 21 . 6 YJc?kt Alts 2ntt.DBr OnItIoNEGAT|VES 6-10 ?-ll Mos. TYPE 3 2 3 4 Y E A R S 70 60 50 40 30 20 t o o The virological evidence of the marked &9p in the dissemination of polioviruses that oecurred 8 to 12 weeks after vaccination is supported by the sero' Trlsra 4.-Poliopirus Anti*rodv Conoersion Rales in Toluca Among " S in gIe-N e garioe i " Doubh-N e gatioe," eoil"' T riplc- Negatioe" Children Ten Weeks After Feeding Singb Dosa of TrtwlcntVaccke Y E A R S 7-il I M6. 60 50 40 30 20 t o o Total { }00 16 88 33 55 5C 6t 1 7 8 9 8 0 8 5 8 3 n t 2 77 ir9 ?{ t? C5 Sil 76 ag tion of all tbree types of poliovirus. The lower ftnal conversion rate for type I may also be related to the fact that we did not use the pH test for low- avidity antibody, which ocmrs more commonly after infection with the type 3 virus. It should also be noted that the second feeding was not a mass feedingbut involved only 44 children under 4 years of age who were &stributed among a large number of families througbout the city. The results might have been even better if nature had been given anotber chance at dissemination of the polioviruses iu the community by feeding all the children under 4 years of age another dose of tbe vac'cine. Figure 5 100 YU 80 2 3 4 Y E A R S TYPE 2 q, = u, o(L (|)(J L(l)(L Fig. S.-Status of poliovirus antibody in two Mexican cities: fop, Toluca-vaecinc given; bottom, Queretaro-no vaccinc givei, Numbers aboi'i age groups in left upper diagram indicate number of children tested. Type 1 . . . . . . , . , . . 2 . . . . . . . . . , . E . . . . . . . . . . , Aug. 7.'59 TYPE w 50 40 30 20 ro o 200 Bulletin of the World Health Organization, 1999, TI Q) 1526 POLIOVIRUS VACCINE_SABIN ET AL. No. 7o J.A.M.A., Aug. 6, 1960 depending on the age incidence of recorded eases of paralytic poliomyelitis or on tlre result of a sero- logic survey in the region, eonstitutes a rational initial approaeh to the eradication of poliomyelitis. The rapid disappearance of polioviruies from To- luea under the conditions of the present study indi- cates that the oncoming generitions of cliildret rvill have to be similarly vaccinated at tlie optimum time during the ffrst 6 months of life, because they will have little or no opporfunity for natural acquis! tion of immunity to poliomyelitis. This study was aid.ed by a grant from tlre National Foundation. Elland and Betlrescla Avenues (29) (Dr. Sabin). T*att,7.-Effectioeness of Ali.quo* of Same Lots and Same Doses of Vaccine Used in Toluca When Gioen to Chtldren of Com.parable Age During Winter and. Spting Months in Northen Climates, Trioalent V ercus Motwoalent (l-3-2) % Developed Iovestisator, tio. ot children ^?:tlt*#"t Place, D{othod Without AEti- Type Age of Childrcu, of -AdBiDis- hody Jor IDdi- and fime of Year tratioq cated Type I Z 8 smorodiDtse\', LeDio. TriraleDt, 10 aegative tor I 89 grad, U. S. S. R.. tested ,l mo. 5{ negath'e tor 2 80 chlldreD's homes, i lp later; all to. 88 legative for 8 7l to ? yr., May, 1958 getber in eornP dllr iDg sutllmer SrDorodiDtse\', leuiD. MoDoYaleut, 147 !.eg8tive for I 9? grad, cbildrer's 4.sk. inter- 68 Degetive for 2 100 hones, up to 3 yt., r'al: tested 67 Degati\r€ for 3 96 Ieb., Marcb, April, oDe molth 19i8 after each type ferliDde, LeydeD, IIol. MoDovaleDt 90 Degative for 1 l{rt, land, lndividual g.yk. inter. lE Degative Jor 2 90 lanril ies, up to 14 val lED€gative lorg 100 yr., .[Isy or Dec., 1957 References l. Sabin, A. B.: (a) Status of Field Trials with Orally Administered, Live Attenuated Poliovirus Vaccine, I, A. M, A. l?l:863-868 (Oct. 17) 1959. (b) Receat Stu&es and Field Tests with Live Attenuated Poliovirus Vaccine, in Live Foliovirus Vaccir:es, Papers Presented anil Discussions Held at First Intemational Conference on Live Poliovirus Vac- c.ines, scientific publication no. 44, Washington, D. C., Pan American Sanitary Bureau, i959, p. 14. 2. Ramos Alvarez, M.; Gomez Santos, F.; Rangel Rivera, L.; and Mayes, O.: Viral and Serological Studies in Chil- dren Immuaized r,vith Live Pohovirus Vaccine-Preliminary Report of Large Trial Conducted in Mexico, in Live Polio- virus Vaccines, Papers Presented and Discussions Held at First Internatjonal Conferenee on Live Poliovirus Vaccines, scientiffc publication no. 44, Weshington, D. C., Pan American Sanitary Bdreau, 1959, p. 483. 3. Plotkin, S. A., and Koprowski, H.: Epidemiological Studies of Safefy and Efficacy of Vaccination with CHAT Stnin of Attenuated Poliovirus in Leopoldville, Belgian Congo, in Live Poliovinrs Vacciaes, Papers Preseoted and Discussions Held at First International Conference on Live Poliovirus Vaccines, scientiffc publication no. 44, Washing- ton, D. C., Pan American Sanitary Bureau, 1959, p. 419. shorvs that with these two feedines of vaccine it was possible to achieve au imm;nogenic efiect \f ithin a few months and without any eoit in paraly- sis, comparable to that achieved naturally only after four years and- at a price of about 56 cases of para- lytic poliomyelitis. The comparable serologic efiectiveness of the same lots and doses of vaccine trsed in h.ivalent or rnonovalent form rrnder di$erent conditions in chil- dren in northerl) climates is slrorvn in table 7. It is evident that vrlren the interrral betrveen the sepa- Trct.r S.-Inc idence of Virtts Recooerg fron Rectal Stoabs in Belatiott to Develapntant of Antibod7-Estimate of Total Infection Rate During Ten-lVeek Period Aftn Single OraI Dose of Triualent Vaccine in Toluca Typc l Typ{i 9 I'Ipe B Cat6gory l:irus Io Rectd Swabs Auroog fhose Who Developed ulti l .ody .,..,...., 19/9{i 33 5Sl08 82 9I/&l F&iled to derrloD aDtibody .. 9/l? 72 3/t{ n UB Estirn&te of total lDiectioD rate {!/ ig E0 67t' i i e? gf/?6 DevcloltnL.xt of aDtibody % eoDrerted at IEdieated Time After Filst Dose, Muds peedirlg SocoDd l)ose. - ^-- }l-csative Ooh' L'hildren First 10 Wk, Next U Wk. - 6 Wh. - Boib Doses .^ 'festerl, .!ug., f959. O(t., 19i9. JaD., l9{i0. Aug., lgi$'rype No. clet.,1959 J{!., t960 reb., lgm rcE., tcorr r .i3 68 7 .' 85 116 2 7 7 8 9 0 1 9 1 $ I . 7 6 { S 3 { 8 ? 2 of the three types of vaccine at intervals of not less than four to six weeks is optirnum during the cold or cool months of the year in areas with good sani- tation and hygiene. Conelusions The results obtained in Toluea, lvlexico, are of signiffcance for the vast majority of the world popu- Iation, among whom immunity to poliomyefitii is naturally acquired during the ffrst few years of life at a varying and somelimes considerable price in paralysis. In such areas the feeding of trivalent vac- cine on two brief occasions at an interval of six to eight rveeks to all children aged under 4 or 5 years, 64 li allcrtrg totol Do. i0fected(dentoustrsted + cstirnsted) 3f/{j e6 {rl/67 94 Sg/i6 92 rate, sequential feedings of the three types was not less than four weeks, tbe antibodv conversion rates were 100% or close to it, r;r,hile lhe mixture of all three types yielded conversion rates of only 82, 80, and'|LVo, respectively, for the three types in a mixed group of single-negative, double-negative, and triple-negative children living under conditions that were conducive to natural spread of the vaccine strains among them. These reiults, as well as other data collected by Russian investigators in 1g5g, in- dicate that the separate sequential administration T,lnr.r 6.-Corn:,ersion Aates at Diferent Times After Feed- ing Two Doses of Tilr:alent Polioairus Vaccine in Toluca Bulletin of the World Healfr Organization, 1999,27 (Z)