Because the sample drawn from private labs is more likely to represent relatively wealthier people and those with health concerns, researchers led by epidemiologist Prabhat Jha also calculated prevalence in the 12 cities based on mortality data for Covid-19 for the same time period. They estimated a prevalence of 43-60% in those over 20 years in these cities, with 9-12 infected adults for every confirmed case.
The findings broadly align with sero-surveys in Delhi and Mumbai, said Sandeep Juneja, a senior professor at the Tata Institute of Fundamental Research who has led Mumbai sero-surveys and is not associated with this study.
The high prevalence rates suggest variants are driving the current surge since widespread infection is usually thought to bring herd immunity. It’s also possible that mild infections last year led to fewer deaths but also a lower protective effect, said Jha, who is director of the Centre for Global Health Research in Toronto. “Low levels of infection may be like a shoddy vaccination that doesn’t really work.”
The study found antibodies peaked in those aged 14, declined by 20, and rose again to peak in the 60-70 year group —-an “M-shaped pattern” that suggests intergenerational transmission, said Jha. In some countries, antibodies are highest among young adults, but in countries with multigenerational homes, like India and Brazil, the entire family gets infected, he noted. People in their 20s, who are most likely to be living away from family, had lower positivity rates.
This is an important finding, said health and development economist Anup Malani, who was not associated with the study. “The role of intergenerational transmission tells us about the value of lockdowns for protecting the elderly,” he said.
Moreover, while seropositivity rates rose in all age groups between July and December, fastest growth was in those under 20 and the 20-44 age group, suggesting younger people were driving transmission.
The study, still in the pre-print stage, tries to throw insight on what Jha calls the “paradox of widespread infection and low mortality” in India. India was among the top in Covid-19 cases last year but had relatively few deaths compared to Europe and America, leading some to suggest there were a large number of hidden deaths. Jha’s analysis of the mortality data in the 12 cities suggests that is not the case in urban areas: the cities were chosen for their good death registry systems, and the mortality data was adjusted for under-reporting. Higher death counts would have translated into “implausibly high” infection rates, Jha says.
Researchers also looked for, and did not find, any associations between seropositivity and migration, age structure of the population, or PM 2.5 levels. They did find a small 6% decline in seropositivity in areas where there had been an increase in childhood vaccination for measles in 2014-15. “It’s possible that kids who were vaccinated were more resistant to Covid-19 and that helped reduce transmission to adults, too,” said Jha. The finding was “speculative”, he noted, but did suggest background immunity may play a role.
Malani, a professor at the University of Chicago who has led serosurveys in India, said while the study sample was not representative of the urban population—being a self-referred group able to afford a private laboratory test—its size and the detailed analysis offers insights on seroprevalence across cities and time.
The study provides more evidence that antibodies wane over time, Malani noted, since seroprevalence rates fell in some cities by end of year.