Experts unsure about the protection period of each vaccine

Pune: Most Covid vaccine frontrunners have thrown up promising efficacy data. But it’s still unclear how long their protection will last.

Health experts are debating the duration — some say it's six months; others say it can be as long as nine months.

But almost all agree it's just too early to tell. Dr K Srinath Reddy, president of the Public Health Foundation of India, said since trials are still ongoing and given the fact the virus hasn't been around for long, it would be difficult to be certain about the duration of vaccine-induced immunity.

He said, “With natural infection, antibody levels fade in three months. Cellular immune response, mediated by T cells, has been reported to last up to six months. But we still need to study if this form of immunity lasts longer.”

In natural infection, the antigenic stimulus varies from person to person and is based on viral load, he added.

A vaccine provides a stronger and consistent boost to immunity because of a standardised level of antigenic stimulus. This is the reason why a vaccine generally provides longer protection compared to the body's own immune system. "But (for Covid) we don't have available data showing how long," Dr Reddy said.

Dr Gagandeep Kang, vaccine expert, said a clear answer will have to until the vaccines are administered. She said, “It is important to remember that vaccines are designed to induce the best possible immune response. We have data that shows immune responses vary according to severity of illness. We do not know if this means a difference in protection so, we'll have to wait and watch as more previously infected people are exposed to subsequent infections."

Former head of epidemiology and communicable diseases at ICMR, Dr Lalitkant said there's still no data on duration of protection. He said that while the makers of the five vaccine frontrunners have released encouraging news, their data will have to be peer-reviewed. "The industry is sharing data. That's positive news, but the details will have to be reanalysed."