India unveiled its injectable, inactivated polio vaccine, heralded as the final onslaught in the war against polio, on Monday — even as the national capital was wrapping up a focused, month-long, intensive surveillance of the virus in a district on its northeastern border. A case of vaccine-derived polio, in the first week of November, of the strongest P2 strain of the virus, had upped the ante in the state polio programme.
This two-and-a-half-year-old child — like the 44 others before her who contracted the virus from vaccines taken to prevent the disease since the launch of the pulse polio programme 16 years ago — underscores the haziness of the routinely-quoted assessments of risk from VDPV, officials involved in the programme say. “In a programme of this scale, running since 1999, the risk of one in 2.5 million children contracting the virus from the vaccine seems very remote. You hear of cases, but never think it can happen under your nose… This is like an earthquake or a hurricane,” said an official who has been involved with Delhi’s polio programme for nearly a decade.
The oral vaccine contains live but attenuated (or highly weakened) forms of the three strains of the polio virus — P1, P2 and P3. These are just enough for the immune system to recognise, and develop its defences against the virus antigens. The process of bolstering immunity involves the body creating antibodies specific to the antigens of the three virus strains. Traces of the attenuated virus are also excreted by the immunised child, while his or her body is developing this immunity.
In rare cases, while the body is developing these antibodies, the attenuated form of the virus mutates to a stronger form — and starts actively circulating in the vaccinated child. Thus the vaccine ends up causing the very disease it was meant to prevent. As the child excretes this virus, it spreads further into the environment.
International agencies like WHO, and countless peer-reviewed studies, have acknowledged this risk from the oral vaccine. But they all state that the numbers of wild, or environmentally contracted, polio virus cases the vaccine has prevented far outweigh these risks. According to the WHO, since 2000, over 10 billion doses of vaccine in three billion children have prevented at least 13 million cases of polio; oral vaccine has helped cut the incidence of the disease by more than 99%. In comparison, during this time, some 24 outbreaks of VDPV took place in 21 countries, and together caused fewer than 760 cases.
The vaccine derived cases are seen most commonly in children with extremely poor immunity and nutritional indicators. The Delhi child was described by officials as “highly immunodeficient”. Harsh Vihar in Shahdara district, where she lives, is highly congested, and characterised by poor sanitation and socioeconomic indicators. The child, now 25 months old, had received around 20 doses of the oral polio vaccine as part of routine immunisation.
In the first week of November, her stool samples tested positive for the P2 strain of the polio virus, confirming the worst fears of field workers. P2, the strongest strain, which was also the first to be eradicated way back in 1999, has emerged as the most common vaccine derived polio strain. There is a proposal to remove this strain from the routine immunisation vaccines from April next year.
The case set off intensive epidemiological surveillance to restrict further spread of the virus. Authorities have collected and examined stool cultures from 36 people in contact with the diagnosed child, and seven samples from multiple sewage sites. All have tested negative for the virus.
International studies have pointed out that the emergence of a vaccine derived case actually reflects poor immunity status of the community as a whole. The attenuated virus survives longer if the body’s immune response is weak, and the longer it lives, the greater the chances of it mutating to a disease form.
The P2 strain this child has can lead to all the symptoms of polio, including paralysis. That the strain could survive in the child long enough to mutate into an active form, is an indication of inadequate coverage of the polio vaccine. In simple terms, had the child been given supplementary doses of the vaccine even after the virus started circulating in his body, its mutation to a full-blown disease could possibly have been averted. Recognising this, the union Health Ministry ordered a round of supplementary polio immunisation in areas in the vicinity of the child, between November 6 and 10, within three days of the diagnosis.
The injectable form of the vaccine, in contrast, uses inactivated forms of the virus. They do not even have traces of the live virus, but can still trigger the necessary immune responses in the body. For total eradication to become a reality, the virus has to be eliminated from the environment, and there has to be an end to vaccine derived cases like this one. The injectable vaccine is expected to help achieve this by removing disappointments like the one in the national capital last month. As an official put it, “It would be nice to live in a world with no risks.”