Letter to the Editor

Are We Teaching Some Wrong Facts Pertaining to Oral Polio Vaccine?

Sir, Park's Textbook of Preventive and Social Medicine 1 states

on page 154: "The vaccine progeny is excreted in the faeces and secondary spread occurs to household contacts and susceptible contacts in the community. Non- immunized persons may therefore be immunized." Thus widespread "herd immunity" results, even if only aproximately 66 percent of the community is immunized3 This property of OPV has been exploited in controlling epidemics of polio by administering the vaccine simultaneously in short period to all susceptibles in a community. This procedure virtually eliminates the wild polio strain in the community and replaces them by attenuated strains. 3 The duration of immunity produced by the OPV is not known, it may possibly even be life long".4

1. Herd Immunity by Secondary Spread

Different studies from 1959 onwards had demonstrated that this herd effect was very low to be of any clinical value, s These included studies by Sabin 6 and WHO. 7

The s tudy conducted by the World Health Organizat ion Collaborat ive Study Group on Oral Poliovirus Vaccine had stated: "These observations cast doubt on the importance of indirect spread of vaccine virus in raising overall levels of humoral immunity to poliovirus types I and 3, at least during the fist 6 months of life, and reinforce the need to provide sufficient number of doses of OPV to all infants. "7

This negligible role of herd immunity is because of very low virus load spread through faeces as compared to the high virus load in the vaccine. The children who had already failed to respond to two or more doses of relatively high titers of vaccine virus (> 105 median tissue culture infective dose of each serotype) would not be expected to seroconvert when exposed to much lower titers (approximately 102 median tissue culture infective dose per gram of stool), s Following is the simple arithmatics of the above equation. 1 lakh type 2 polioviruses and 6 lakh type 3 polioviruses i.e. 17 lakh polioviruses in each dose of OPV administered. One gram of stool contains about 100 polioviruses, TM i.e. 17 kg of fecal matter will provide same quantity of polioviruses as are contained in one dose of OPV.

2. OPV During Epidemics

Park's textbook has stated that this property of herd immunity helps in controlling the epidemics. The study by Nightingale 3 quoted in the textbook x had stated: "The

OPV was favored because of its ease of administration (oral instead of injected), expected long lasting immunity, and the production of bowel immunity, which in turn would lead to interruption in the chain of transmission of wild viruses from the population". In the WHO Bulletin, again quoted in the textbook, Melnicld had elaborated the issue further .... because the live vaccine viruses become established quickly in the alimentary tract of the vaccine recipient, they are capable of blocking infection with epidemic virus strains within a matter of days even before the vaccine-induced antibody becomes fully effective". This property of quick establishment in alimentary tract makes OPV superior to IPV during epidemics. But the Park's Textbook had erroneously attributed this to herd immunity and stated: "This property of OPV has been exploited in controll ing epidemics of polio by administering the vaccine simultaneously in a short period to all susceptibles in a community."

3. Vaccine Polioviruses will Replace wild Polioviruses

Unlike Cl tetani which are found in the intestines of many herbivorus animals, polioviruses are not part of intestinal flora of human gut, and chronic carrier state does not occur. Human gut acts as conduit for polioviruses for fecal to oral to fecal circulations.

During the transit in human gut the wild or vaccine polioviruses may replicate for 4-8 weeks, may cause disease or induce immunity respectively and then depart from the human gut. The author failed to find any study or reference that vaccine polioviruses replace wild polioviruses, and raised the issue with the virologists, and the experts had stated: "It is primarily a question of which virus got access to the host first. If the child is first exposed to the vaccine strains then the vaccine strains would cause gut immunity and systemic immunity against poliovirus, causing large proportion of the previously susceptible population to be thus immune, will break the natural cycle of transmission of wild poliovirus at a community level. In a given individual the vaccine virus if given after the exposure to the wild virus will not replace the former. "9

Thus repeated doses of OPV are given not to replace wild polioviruses, but to replace susceptible unimmunized population with immunized population, so that wild polioviruses may not find suitable host to replicate and this will eventual ly abolish the wild polioviruses circulation.

OPV provides a negligible herd immunity by secondary spread, but it has been exaggerated; and

Indian Journal of Pediatrics, Volume 69~December, 2002 1101

Yash Paul

vacc ine p o l i o v i r u s e s do n o t r ep lace w i l d p o l i o v i r u s e s , so a n o n - e x i s t i n g p r o p e r t y has b e e n a t t r i bu t ed to OPV. It is

m o r e t h a n t w o d e c a d e s n o w t h a t e r r o n e o u s i n t e rp r e t a t i ons h a v e no t b e e n rect i f ied.

Yash Paul, A-D-7, Devi Marg, Bani Park

Jaipur - 302016, India. E-mail : devisharan_@yahoo.com

REFERENCES

1. Park K. Development of immunity, In Park's Textbook of Preventive and Social Medicine. 16th edn. Jabalpur; Banarsidas Bhanot, 2000; 151-157.

2. Ichout BD. Med Int 1988; 53 : 2189-2191. 3. Nightingale EO. Recommendations for a national policy on

poliomyelitis vaccination. N Engl J Med 1977; 297 : 249-253. 4. Melnick JL. Advantages and disadvantages of killed and live

poliomyelitis vaccine. Bull WHO 1978; 56 : 21-38. 5. Paul Y, Priya. Current Controversies in Immunization. tn

Gupta S, ed. Recent Advances in Pediatrics, special Vol 11, Community Pediatrics, New Delhi, Jaypee Brothers. 2002; 65-86.

6. Sabin AB, Michaels RH, Spigland I et al. Community wide use of oral poliovirus vaccine. Am J Dis Child 1961; 101 : 546-567.

7. World Health Organization Collaborative Study Group on oral poliovirus vaccine. Factors affecting the immunogenicity of oral poliovirus vaccine. A Prospective Evaluation in Brazil and the Gambia. ] Infect Dis 1995; 171 : 1097-1106.

8. Onorato IM, Modlin JF, McBean AM, Thomas ML, Lososky GA, Bewnier R. Mucosal immunity induced by enhanced potency IPV and OPV. J Infect Dis 1991; 163 : 1-6.

9. Paul Y, Sridharan G, Abraham P. How do the vaccine polio vi ruses replace the wi ld pol io viruses? Indian J Med Microbiology 2002; 20 : 56,

1102 Indian Journal of Pediatrics, Volume 69--December, 2002