Jaipur: Nearly 68%, or two in three Indians, have antibodies against Covid-19, the latest national Covid-19 seroprevalence survey has found. While "this is some good news, this is not the time to celebrate", cautioned Giridhar R. Babu, professor and head of life-course epidemiology at the Public Health Foundation of India (PHFI). Millions of Indians are still susceptible to the virus, the survey has shown, in addition to vast underreporting of cases, especially in rural India.
Vaccinating more people, including children; better testing; and precautions such as mask wearing and physical distancing are the only ways out of the pandemic, health experts say.
The survey, whose results were announced on July 20, checked for antibodies among the unvaccinated, those who have received one dose of the vaccine, and those who have received both doses. In those who were unvaccinated, 62.3% had antibodies, the survey found; 81% of those with a single dose of the vaccine and nearly 90% of those with both doses had antibodies. So far, 9% of the Indian adult population has received two doses of the vaccine and 35% has received one dose of the vaccine, shows our analysis of data from covid19india.org.
The survey was conducted across the same 70 districts in 21 states as its previous three rounds. Unlike the previous surveys, when only the adult population was included, this latest round also included children between the ages of six and nine years.
"Since the Delta variant is highly contagious, the fact that about 32% of Indians remain to be infected is sufficient for a new wave to be set off in the future, or at least for case numbers to remain roughly at the same point for a while, neither increasing nor decreasing," said Gautam Menon, professor of physics and biology at Haryana-based Ashoka University.
As many as 400 million Indians still do not have antibodies and are susceptible to the virus, Balram Bhargava, director general of the Indian Council of Medical Research (ICMR), said at a press conference on the serosurvey on July 20.
"However, it is likely that a third wave will not have the ferocity of the second because of the large numbers with a prior infection," said Menon. New infections are likely to be dominated by pockets of those who were not infected earlier and who have not yet got vaccinated, he explained.
India has reported 38,328 new Covid-19 cases and 528 deaths from the disease on July 19. Though this is about 9.5% of the 400,000 cases India had in one day at the peak of the second wave of Covid-19, it is still 41% of the peak of the first wave when India had around 94,000 cases in a day.
Why serosurveys matter
Seroprevalence tests help estimate total infections, including mild and asymptomatic cases, that might not have been tested, says this paper on the third seroprevalence survey in India.
That the prevalence of antibodies is almost the same in rural (66.7%) and urban areas (69.6%) but a far lower number of cases were reported from rural India shows that testing in rural India needs to improve, said Babu.
These surveys also tell us how the disease is spreading over time, said Menon. From the first serosurvey in May and June 2020, when one in 140 adults had antibodies for the virus, two in three Indians now have antibodies either because of prior infection or vaccination.
At the same time, "all said and done, this serosurvey has information on only about 70 districts of the 700-odd districts in the country", said Menon. "There is heterogeneity from state to state and district to district," Bhargava said, at the press conference, adding that states and districts without antibodies run the risk of infection waves.
"We need more granular information at the level of each district as well as information about reinfections and vaccine escape, if we are to gauge the impact of Covid-19 in the future," said Menon. "The government should prioritise making this information available."
So, is herd immunity unreachable?
Herd immunity is the "epidemiological term applied to the observation that not everyone in the population needs to be vaccinated against a disease for the protection to extend to everyone, but only a sufficiently large fraction", which is called the herd immunity threshold, Menon explained. It is technically incorrect to assume that everyone who has had a prior infection can be counted towards this threshold, he said.
This is because the virus is changing and there are re-infections, especially with the Delta variant, health experts said. In addition, you would need to know "the reproductive number (R, the number of people infected by one patient, on average) of the virus and there should not be any superspreading events," to estimate the herd immunity threshold, said Babu. To know the herd immunity threshold for Covid-19, "we need to understand the immune response to SARS-CoV-2 better than we do at this point", Menon said.
Babu pointed out that in the UK and Israel, where higher seroprevalence and vaccination levels are reported, cases are still increasing. Hospitalisations and numbers of deaths are the only markers to know if the population is protected. These numbers in the next wave will be completely dependent on how many people will be vaccinated, he added.
Why were antibodies not present in everyone vaccinated?
Of those who had received two doses of the vaccine, 90% had detectable antibodies; of those with a single dose, 81% did, the survey found. Nearly 85% of healthcare workers had antibodies while 10.5% had not taken any vaccine, the survey found.
"It is a bit of a mystery as to why about 10% of those doubly vaccinated show no antibodies. But the absence of detectable antibodies need not correlate to an absence of protection in the long term," said Menon. Protection from disease need not solely come from antibodies but from other parts of the immune system, such as T-cells that are involved in maintaining a memory of a past infection ['immunological memory'], he explained.
Even in cases when a person recovers from Covid-19, antibodies are not always detected and they also wane over time, Babu said. Still, because of the immunological memory, if a live virus attacks, the body will be able to fight it because of the memory of the response, explained Babu.
Should children be vaccinated?
One long-standing assumption has been that children are not infected as much as adults, but the survey finds this to be untrue. Over half (57.2%) of children, aged six-nine years, had antibodies, while 61.6% of those between 10-17 years had antibodies. Because children are not yet being vaccinated in India, this shows that children were exposed to the virus.
The data also show that "there is no reason to believe that children should be more at risk in any future wave of the disease, or indeed, were more at risk in the second wave," said Menon.
But "we do not yet know how this impacts children in the long run and how common are cases of Multi-Inflammatory Syndrome in Children (MISC)", said Babu. MISC results in inflamed blood vessels throughout the body which can limit blood flow, damaging the heart, kidneys and other organs, after a child has recovered from Covid-19. This is why India should immediately conduct bridging studies for the vaccines that are being used in children internationally, such as the Pfizer-BioNtech vaccine. Trials of the vaccine that uses the inactivated virus, such as those made by Bharat Biotech, need to be conducted fast in India, said Babu. Bharat Biotech's clinical trials to evaluate the safety and efficacy of its vaccine in children aged between two and 18 years were approved in May 2021. At the same time, a surveillance system should be set up to track the effects of Covid-19 on children, Babu added.
Menon disagreed. "There seems to be no need to vaccinate younger children. The risk of disease, especially severe disease, rises sharply with age, and is small (although not zero) for children less than 18," he said. Vaccines should focus on older people and immunocompromised individuals who are at larger risk of serious disease, he said.
What precautions should people still take?
"We must maintain Covid-appropriate behaviour and community engagement," Bhargava said at the press conference. Governments should track patients with severe acute respiratory illness (SARI) to identify any increase in such cases or Covid-19 cases among SARI patients, Bhargava said. They should also identify clusters of Covid-19 cases that develop suddenly, and track if the severity or symptoms of the disease are changing, he added.
"Indians should continue to maintain their vigilance, taking care with masking and avoiding dense crowds, especially indoors," said Menon. "We should continue with fast and wide vaccination coverage," said Babu.
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