The number of reported Covid-19 positive cases in India has crossed the 10 million mark. More than seven percent of the population was found to have developed Covid-19 antibodies as per the second nationwide survey carried out by the Indian Council of Medical Research in August-September when the number of reported positive cases were ~2.5 million. This translates to an “infected to positive” ratio of 40 to 1, with majority of them being asymptomatic.

The projected proportion of people with evidence of prior infection will near ~30 percent as positive cases quadrupled from mid-August to end-November. Current levels of containment measures such as face masking, hand hygiene, social distancing, and case isolation have brought down reproduction rates to a level (~1.1 to 1.2) where herd immunity in 30 percent of the population will be needed to arrest the pandemic. This might explain the declining trend being witnessed in daily cases across many states in the country. Any relaxation in present levels of containment measures carries a risk of resurgence of infections as reproduction rates begin to rise. This is being witnessed in the National Capital Region.

As we prepare to prioritize categories of people who will be vaccinated on priority, the question that remains to be answered is whether people who have evidence of prior infection need to be vaccinated. The proportion of people with prior infection will rise further by the time the vaccine administration begins. It is still not clear whether vaccine trials are restricted to people with no evidence of prior infection or whether they are also being tested in people with prior infections. What is also not clear is how long will people with prior infection stay immune to reinfection. Sporadic case reports across the world do point to the possibility of second infection. Similarly, studies demonstrated faster decline in antibody levels in people with no or mild symptoms of Covid-19. However, it doesn’t mean that the people will not have immune protection as they have exhibited prompt immune response triggered bythe memory of prior infection. In addition, other forms of immune response, including that from cell-mediated immunity (as opposed to antibody mediated immunity) do offer protection from reinfection.

Given the expected vaccine supply constraints, prioritization policy should consider the status of prior infection. Those people with evidence of prior infection may be excluded from the priority list until further evidence emerges on the duration of protection a prior infection or vaccine offers. This brings in another logistics issue of testing for prior infection in those who did not have prior evidence of the virus. Testing for the presence of neutralizing antibodies against Covid-19 in all people being prioritized for vaccine will be expensive and challenging. Policy makers and scientific community advising them should actively seek information from companies undertaking vaccine trialson the results of vaccination in participants with evidence of prior infection in formulating the vaccine policy.

Dr. Krishna Reddy Nallamalla

Country Director, ACCESS Health India