‘Dangerous’ Assumption: Herd Immunity & COVID Sero-Surveys

In the last 10 days, cities such as Delhi, Ahmedabad and Mumbai have released findings from their respective serosurveys, carried out to estimate the spread of COVID-19 in population groups by detecting the presence of antibodies in individuals.

The Delhi survey by the National Center for Disease Control found that 22.86% of the 21,387 people tested in 11 districts showed signs of past exposure to the novel coronavirus - which translates to nearly one in five residents.

On the other hand, Mumbai’s serosurvey indicated that about 57% of the participants in slum areas had developed antibodies to the virus, as opposed to the 16% in non-slum areas.

These results, along with a noticeable dip in the number of cases Delhi and Mumbai have been registering lately, have added fuel to the belief that they may, in fact, be heading towards herd immunity. Chief Minister Arvind Kejriwal spoke about this explicitly in Delhi’s context. But is such an extrapolation of results logical? Is this how herd immunity works?

Understanding Herd Immunity, and All That It Isn’t

'Herd immunity' ensures that a large percentage of the population becomes immune to infections like COVID-19, which breaks the transmission chain and protects the remaining people within the group to also not get infected. This can be achieved either through exposure to the infection or via vaccination. FIT had explained in detail what this means here.

Dr K Srinath Reddy, President of Public Health Foundation of India (PHFI), in an earlier interview, told us that a lot of misunderstanding surrounds this concept. The threshold of herd immunity, i.e the number of people who need to be infected (and have antibodies) to achieve it, depends on the rate of transmission (RO) of a particular virus. The greater the RO, the greater the immunity threshold.

For instance, the R0 for polio and smallpox was 5-7 and the herd immunity threshold was 80-85 % of the population. Since it was a very severe disease, it took us 100 years to achieve this - not through acquired immunity via infections, but instead, through vaccines.

For COVID-19, this threshold is unlikely to be less than 50% - and even these are just estimates. We don’t know for sure how many people in a particular area need to have acquired antibodies to achieve herd immunity. Naturally, if we don’t know that, we cannot say if a particular city is there yet.

Now, what do the serosurveys do?

These surveys at best indicate the possible spread of the disease in the community and help authorities come up with better strategies. They show us the presence or absence of antibodies in a person - they do not tell us the amount, and they cannot identify if these are ‘neutralising’ antibodies (which specifically target the pathogen in concern).

To sum it up:

1. Since it is not possible to test everyone, it is not clear how many people in the population are infected. Even among the sample chosen, there is the possibility of false positives due to antibodies from other coronaviruses in the blood.
2. Two, the serological tests cannot indicate the quantity of antibodies in specific individuals to know if they have enough to be immune to a virus.
3. Add to this what we discussed about herd immunity threshold: We don’t even know the number of people who need to be infected to achieve herd immunity against COVID-19, or the amount of antibodies needed for it, or even whether the mere presence of antibodies is enough to assume immunity in this case.

Another Caveat: Localised Herd Immunity

Moreover, herd immunity is not an individual characteristic. It depends on the entire ‘herd’ being ‘protected’ from the virus. Since serosurveys focus on particular districts, extending their results to the entire population - when the chances of a ‘uniform’ infection spread right now are minimal - and becoming complacent would be extremely risky.

For instance, even if we assume Mumbai’s slum dwellers have reached herd immunity, if one of them travels to another region where the transmission is still taking place, they could get infected - just like all others living there. Herd immunity, therefore, applies to closed population groups. Free movement of people would put them at risk of infection.

Dr Reddy had explained: “It is herd protection and not herd immunity. Let’s say people 50-60 percent of people in Delhi are infected and develop antibodies and immunity. This would also protect the remaining people in Delhi who have not been infected yet. But suppose one of these uninfected persons (who is protected in Delhi) travels to Ranchi or Raipur, where the infection rate has only been 20-30 percent. That community does not have herd immunity to confer to this person, which makes the Delhiite still vulnerable. So it is the herd that has protection characteristics if it is stable, but not the unprotected individuals of the herd if they travel to another herd.”

Decoding The Links: COVID-19 Antibodies, Immunity & Herd Immunity

Associating the presence of antibodies to immunity against COVID-19 is another overstretched link, experts believe.

“We do not know to what levels these antibodies will confer immunity or what duration that immunity will last. For instance, influenza immunity does not last long. We also need to know if we need cell immunity or whether these antibodies are enough.”

Dr K Srinath Reddy

Then comes the question of reinfection. There is scattered evidence that antibodies in mild to moderate cases last for only a few months - which would mean these individuals would be susceptible to a second wave, as we have also seen in specific people getting infected ‘again’.

For instance, a report in The Guardian from 12 July quoted a King's College London study that found a steep drop in antibody levels in patients three months after recovering from COVID-19.

If immunity itself is short-lived, herd immunity as a defence doesn’t work.

With these questions still remaining about the virus, its spread and immunity, would it be wise to assume we are achieving herd immunity, and by extension, returning back to ‘normal’?

Dr Shahid Jameel, a virologist, warns against such complacency and says it could be quite ‘dangerous’.

“Moving towards herd immunity is an incomplete statement. We all know we will eventually reach that phase. But till the time we don’t know the specifics such as the percentage of the population that needs to be immune or how long this ‘immunity’ lasts, we are not out of the woods. Look at Kerala for instance. The state is now seeing another wave of infection.”

Dr Shahid Jameel

The devastating effects of the contagious virus, especially on people above the age of 60 and those with comorbidities, is reason enough to continue to take its containment seriously.

In Dr Reddy’s words, “With multiple questions hanging around this new virus, I don’t think we can place our bets on this herd immunity to get us out of trouble.”