The survey, conducted by the Indian Council of Medical Research (ICMR) to estimate how widely the virus has spread, however, shows that more than 400 million Indians, especially children, remain vulnerable to covid-19.
Still, the good news is that India’s population is nearing herd immunity, a stage at which the virus can no longer spread quickly. The latest survey, conducted between June and July, showed that 67.6% of the population was exposed to SARS-CoV-2, a massive increase from the 24.1% people found with covid antibodies in the January survey.
“The implications of the serosurvey show there is a ray of hope, but there is no room for complacency. Non-essential travel must be discouraged,” said Dr Balram Bhargava, director general, ICMR.
Dr Lalit Kant, an epidemiologist, cautioned that the serosurveys only indicate the level of exposure to the virus but has little relevance to herd immunity because of re-infections.
“What we know from the available research is that antibodies against covid-19 last for around 9-10 months. After this, immunity wanes; a person is susceptible to the virus again,” Kant said.
Bhargava also warned that mathematical models of virus transmission indicate there are several conditions under which a serious third wave could occur. “There are four potential mechanisms for a third wave: waning immunity that restores previously exposed individuals to a susceptible state, the emergence of a new viral variant that is capable of escaping immunity, the emergence of a new viral variant that is more transmissible, and premature release of lockdowns,” he said.
ICMR’s latest serological survey was conducted on more than 28,975 individuals, including adults and children (aged 6-17 years), apart from 7,252 healthcare workers in 70 districts where earlier three rounds were also conducted.
Among the population covered under the fourth survey, 10% were aged between 6-9 years, 20% were aged 10-17 years, and people aged 18 years and above constituted 70% of the survey sample.
More than half of the children surveyed (6 -17 years) were seropositive, with the seroprevalence similar in rural and urban areas for all age groups, the survey showed.
The seropositivity has been increasing in consecutive serological surveys. The first one conducted in May-June 2020 found 0.7% seroprevalence. The second survey, conducted in August- September 2020, found 7.1% seropositivity, and the third survey carried out in December 2020- January 2021 found 24.1% of people with antibodies.
“In the fourth round of the national serosurvey, the overall seroprevalence is 67.6% in the entire population. Among adults in the survey, 62.2% people were not vaccinated, while 24.8% had taken a single dose of covid vaccine and 13% were fully vaccinated,” said Bhargava.
The government has asked states to ensure full vaccination of all healthcare workers and accelerate vaccination coverage in vulnerable population groups. The government further said people should ensure adherence to non-pharmaceutical interventions. “The governments should continue tracking covid infection in severe acute respiratory infection (SARI) cases district hospitals and identify clusters and clinical severity while the Indian SARS-CoV-2 Consortium on Genomics is tracking variants of concern,” said ICMR chief.
“Eighty-five percent of healthcare workers had antibodies against SARS-CoV-2, and one-tenth of the healthcare workers were unvaccinated. It is very important to accelerate covid vaccination,” said Bhargava.
With a growing debate whether India is inching towards herd immunity with more and more people getting infected, public health experts have said a larger population having antibodies doesn’t ensure protection against the virus.
Public health experts have further called for larger population surveys for a deeper understanding of coronavirus infection and the antibody response.
“Larger surveys are needed for more clarity. People should continue to remain cautious because we are still unaware of the incidence rate of different variants,” said Dr Rahul Tambe, senior consultant, internal medicine and infectious disease, Nanavati Max Super Speciality, Mumbai. “Presence of antibodies doesn’t guarantee protection against covid.”
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