Pune, Sep 8 (FN Agency) Renowned epidemiologist Dr Naresh Purohit on Wednesday stated that the mythical aim of herd immunity is no longer in play as Delta variant has shown to be roughly 60 per cent more transmissible than the alpha variant of the SARS-CoV-2 virus, and up to twice as infectious as the original strain that emerged in late 2019. Dr Purohit, an advisor for the National Immunisation Programme, was speaking at a webinar organised on the topic of herd immunity. He said that the more effective the virus becomes at infecting people, the higher the herd immunity threshold becomes. This has altered the trajectory of the pandemic, meaning even higher numbers than originally thought will have to be fully vaccinated to achieve any sort of herd immunity, he explained.
He continued that the percentage of people needing to be vaccinated against Delta at 90 percent or above to achieve herd immunity, something that may not be possible given the challenge that the vaccines are less effective against the Delta variant than they are against the original variant and the Alpha variant. He further said that the issue of herd immunity is compounded by the emergence of mutations like the Delta variant which reduce the immune response after vaccination. Also the duration of protection provided by the antibodies is uncertain. He averred that according to the World Health Organisation, herd immunity is the indirect protection from an infectious disease that happens either through vaccination or exposure to the previous infection. As per the ICMR’s fourth sero-survey, conducted in 70 districts in the last week of June and first week of July this year, the prevalence of antibodies against Covid-19 was 62 per cent in the unvaccinated people, 81 per cent in those with one dose of vaccine and 90 per cent in those with both doses. Madhya Pradesh reported the highest seroprevalence of 79 per cent followed by Rajasthan and Bihar. The lowest seroprevalence was reported from Kerala (44.4 per cent).
The survey looked at antibodies against Covid among the general population. He observed that the data suggests India is heading towards herd immunity, there are a number of caveats. The survey was conducted in less than 10 per cent districts and enrolled less than 30,000 individuals. Different states are seeing different trajectories of Covid-19 cases, hence data will vary from state to state. Even the vaccination coverage is different. Till the beginning of September, around half of the 18-year plus population has received at least one dose, with Himachal Pradesh having over 95 per cent of its population covered, and UP and Bihar only 36 per cent. Experts in the webinar opined that the waning immunity after vaccination or natural infection, and the fact that a large population have a long way of achieving the vaccination targets impending Covid third wave cannot be ruled out. They added that the situation is made worse by many people dropping out after the first dose. As many as 1.6 crore people have not received the second dose of the Covishield vaccine even four months after the first one. These include the elderly, hospital workers and frontline workers, groups which are vulnerable. These numbers may be an underestimate as the recipients of Covaxin are not included. Experts said that the the next few months are crucial. The way forward is to encourage vaccination and adhere to Covid-appropriate behaviour.