Indian Pediatrics 2000;37: 794-795

Q. There are occasional reports of recovery of vaccine virus from non vaccinated children, which means that there is some circulation of vaccine virus following oral polio vaccination. Kindly comment on the extent of this circulation of vaccine virus and it’s role in pulse immunization.

Similar spread of vaccine virus is suspected in case of other live vaccines also. Hence kindly clarify whether it is safe to give MMR vaccine to a child when there is a pregnant lady in the house.

A. Santhoshkumar,
Assistant Professor,
SAT-Medical College,
Thiruvananthapuram,
India.

Oral poliovaccine consists of over a million infectious virus particles. Immunized children who have established vaccine virus infection in the gastrointestinal tract may excrete vaccine viruses for a few weeks. However, the probability of another person getting infected from this source is very low, partly because vaccine polioviruses possess very low infectivity. That is the reason why we have to hike the virus content of the vaccine to one million type 1 virus, 100,000 type 2 virus and 300,000 type 3 virus. Yet, occasionally vaccine virus infection may occur in a nonimmune child in the household. Had this been a more frequent phenomenon, we could have achieved excellent immunization coverage by feeding OPV to just about half the children in a given community. That such circulation is infrequent and unreliable is the reason why we have to feed OPV to virtually 100% of children, and that too repeatedly.

The rubella virus in MMR vaccine does appear in the throat of some of the immunized children. Such virus shedding is intermittent and of short duration; it is extremely unlikely to act as a source of infection for any one else, even in the same household. Therefore, no untoward incident such as infection of a pregnant woman, from an immunized child, has been documented. Even in the very unlikely event of such transmission, no adverse effect due to rubella vaccine virus on the developing fetus has been documented. Yet, caution is recommended to avoid rubella immunization of children when a member of the household is in early pregnancy. This is based only on theoretical grounds.

Let me once again state that vaccine polioviruses do not circulate to any significant degree in the community during or after pulse immunization with OPV. The purpose of pulsing vaccine is not for achieving circulation or ping-pong transmission between children, but to create mucosal immunity in children simultaneously. OPV induces very poor mucosal immunity and whatever mucosal immunity is induced appears to be short-lived. Therefore, the cumulative mucosal immunity of a very large proportion of children due to 3 doses of OPV given in pulse manner will result in the interruption of wild poliovirus trans-mission among the children in that community. If, only 2 doses are given by pulse, the cumula-tive mucosal immunity will not be sufficient to break the chain of transmission. For that purpose it will take 3 doses.

In summary, live virus vaccines such as OPV, MMR and varicella vaccine, are very poorly infectious from the vaccinated child to others in the vicinity.

T. Jacob John,
Emeritus Medical Scientist (ICMR),
2/91 E2,Kamalakshipuram,
Vellore,
Tamil Nadu 632 002, India.