A recent sero-survey in Delhi found that one in four people were exposed to the deadly coronavirus. Worse figures were reported from Mumbai, where more than half the slum population was found to be infected with Covid-19. But the fact that most of these cases were asymptomatic or immune to the virus led to hopes that communities are gradually developing natural herd immunity against the fatal disease.

Herd immunity is developed either naturally or through vaccination. At present, two Indian vaccine candidates are in different stages of human trials, and one is in pre-clinical trial phase. Health experts are divided; one group says it's only a matter of time that Indians develop herd immunity, while the other group, more sceptical, prefers to wait for a vaccine.

"It could be possible, especially if you see the latest sero-surveys in Delhi and Mumbai, that people have developed immunity. Though data is not enough, and we need more and more sero-surveys to evaluate any possibility of herd immunity. I think only a sero-survey could reveal how many people have been exposed to the virus and are not showing any symptom," leading epidemiologist Dr Jayaprakash Muliyil.

"It could be possible, especially if you see the latest sero-surveys in Delhi and Mumbai, that people have developed immunity. Though data is not enough, and we need more and more sero-surveys to evaluate any possibility of herd immunity. I think only a sero-survey could reveal how many people have been exposed to the virus and are not showing any symptom," leading epidemiologist Dr Jayaprakash Muliyil.

In Delhi, the National Centre for Disease Control, a Union health ministry body, had conducted sero-survey from June 27 to July 10 to find the extent of corona infection. Initial findings revealed 23.48 per cent of Delhi's population had already been exposed to the virus, and most of them were asymptomatic.

Similarly, serological survey data from Mumbai showed that 57 per cent of slum and 16 per cent non-slum population were infected, but they had developed immunity against the virus.

Dr GC Khilnani, a former pulmonology department head at New Delhi's All India Institute of Medical Sciences (AIIMS), said we are not even closer to herd immunity. Even the recent surveys in the two megacities did not show wide sero-prevalence in the population, necessary to achieve herd immunity, he said.

"For herd immunity, 60-70 per cent of the population should be infected (clinical or sub-clinical, meaning without symptoms). We are nowhere near it," Khilnani told.

Sero-prevalence tests map the extent of infection in the population. It looks for antibodies against coronavirus in blood cells. If antibodies exist, it means a person was infected at some point with or without symptoms.

"Herd immunity refers to the proportion of population that needs to be immune to a virus to provide indirect protection to people in that group who are not immune to it. This can be achieved by two ways - either a large proportion of the population gets infected naturally (in the case of Covid-19, it would be over 70 per cent), or through a protected vaccine," Dr Priya Luthra, principal investigator in viral diseases research in New York's Trudeau Institute told media.

Nearly 25 countries, including India, are facing a high number of cases. In India, the coronavirus pandemic has spread from big cities to small and rural districts.

Different people with Covid-19 can have very different outcomes depending upon their immunity system. Most patients have reported mild or no symptoms. Experts say nearly 15-20 per cent need hospital care and only up to 5 per cent of all symptomatic patients need critical care.

Most experts agree India is nowhere close to achieving herd immunity through natural infection, and the best bet is a vaccine. "Given the complicated immune responses and unpredictable course of Covid-19, it is hard to say if natural infection would be able to get us to herd immunity, and therefore, our best bet is a vaccine," Dr Luthra added.

Nearly 190 coronavirus vaccine candidates of 12 countries, including India, are in the race to counter the deadly Covid-19 that has infected 2.1 crore people globally and claimed 7.5 lakh lives in the last eight months.