IMAGE: Covid-19 vaccine being administered at a hospital in Bikaner, Rajasthan. Photograph: Dinesh Gupta/ANI Photo.
India’s high seroprevalence of over 67 per cent suggests that a significant part of the population has Covid antibodies but the government is being cautious about making any herd immunity claims. (Seroprevalence is the percentage of individuals in a population who have antibodies to a pathogen; and herd immunity is when a large portion of a population becomes immune to a disease, making its spread from person to person unlikely.)
While some experts believe that a large part of India is still vulnerable to the virus that could trigger a third wave, others feel the country is not likely to be engulfed by another deadly wave unless a new variant emerges.
The fourth national serosurvey to test for Covid antibodies in people released by the Indian Council of Medical Research (ICMR) on July 20 said that two-thirds of India’s population had the antibodies, either due to past infection or vaccination during June-July -- when the second wave had battered India.
“Some states have a lower contribution to this calculation,” says Samiran Panda, head of epidemiology at the ICMR. “There is a tremendous amount of heterogeneity in states. A state where the second wave did not attain a big height can still face a third wave,” he adds.
Eight states, for instance, have been seeing a higher R0, or reproduction number, which indicates the average number of new infections caused by one infected person. Of these, Himachal Pradesh, where tourists are thronging in large numbers, and Jammu and Kashmir have recorded the highest R0.
Epidemiologists believe that the rising cases in Kerala, which has been contributing almost half of daily Covid infections, is due to lower seroprevalence that has left a larger population still vulnerable. The latest serosurvey showed the presence of antibodies in just 44 per cent of people in Kerala -- the lowest.
Some experts, however, believe that another wave is unlikely, especially because of the high numbers that have already been infected by the highly transmissible delta variant.
“Delta cannot cause another wave,” says T Jacob John, former head of the Centre for Advanced Research in Virology and retired professor at Christian Medical College, Vellore. “A hypothetical new variant may cause it. Whoever predicts a third wave is predicting a third variant. That is astrology; it is a pseudoscience,” he adds.
John estimates that actual seroprevalence is likely to be around 75 per cent. “Serosurveys always underestimate the true picture. Antibodies below a certain threshold, for instance, do not get counted.”
The data, some experts also say, points to undercounting of cases, especially in states with highest seroprevalence: Madhya Pradesh (79 per cent), Rajasthan (76 per cent) and Bihar (75 per cent). The health ministry had, however, explained that the detected number of cases are usually lower than the actual number of infections all over the world due to a large number of asymptomatic cases.
Contrary to government calls of caution that the second wave is still raging in India, John says it was over a month ago and the daily case count has been stable between 40,000 and 45,000 for more than a month. “We are on the calm surface of a lake, not a wave. If a number is steady for over a month, then we have to be gullible to believe that the wave is continuing,” John says.
However, cases have been rising in Kerala and the Northeast. “Northeastern states are experiencing a third wave. We cannot say India is experiencing a third wave,” Panda says.
The serosurvey also showed high presence of Covid antibodies in those vaccinated with one dose (81 per cent) and those fully vaccinated (89 per cent).
There are, however, concerns that over 10 per cent of those tested did not show any antibodies despite being fully vaccinated. Experts say this is a subject for a separate study as it may indicate that some people are getting a cell-induced immunity instead of the antibodies. (Cell-induced immunity is T-cell response that eliminates the pathogen in the cell itself, providing long-term protection.)
According to scientific online publication Our World in Data, around 8 per cent of India’s population is fully vaccinated against Covid, while 20 per cent have received the first dose.
ICMR has stressed that the national serosurvey numbers are not representative of the situation in the states, which would require a different sampling. Cities like Delhi are unlikely to see another big surge, says Panda, adding, “Delhi has seen four waves already. India is a big country. All states do not behave uniformly.”
He, however, says that if states lift restrictions prematurely and throw all caution to the wind, then a third wave can begin just as the second wave is descending. This is precisely what had happened as the first wave receded. That time, too, experts had said India might escape a second wave. The reality was quite different.
States, therefore, have been asked to conduct their own serosurveys to understand the granular situation and take policy decisions accordingly. “The situation in states such as Maharashtra is tremendously different from district to district,” Panda says. “Local surveys will provide important insights.”