Pune Serosurvey: Does High Prevalence Say Anything About Herd Immunity?

Every new release of seroprevalence survey data in India sparks enthusiastic queries about so-called herd immunity.

The question is asked — with such high numbers, are we there yet?

That tradition has continued with the Pune serosurvey, which has pegged virus exposure in people across five “prabhags” or subwards at a high 51.5 per cent.

The prevalence, specifically of immunoglobulin G antibodies mounted in response to the entry of SARS-CoV-2 in the body, is significantly higher than that registered in the Delhi (23 per cent) and Mumbai (36 per cent) surveys.

So, are we moving towards herd immunity in Pune?

“People have been misinterpreting the data,” Dr Aarti Nagarkar, the principal investigator for the Pune survey, told Swarajya.

“This data does not say anything about herd immunity.”

It’s hard to make an inference about herd immunity from the early Pune data, because, it is unclear whether the antibodies detected in blood samples of survey volunteers accords them meaningful protection from the virus.

What proportion of these antibodies are 'neutralising' antibodies that can bind to viral proteins and guard against infection?

That is unclear.

Say, if at some point, it does come to light that people have these neutralising antibodies and are protected, more questions follow.

For instance, how long does this protection last before infection is back on the table?

In the case of the original SARS coronavirus, most infected people lost their neutralising antibody protection after the first few years, according to a Nature report, but people with severe infection were said to test positive for antibodies even 12 years later.

It is yet to be determined, conclusively, how immunity plays out in the case of SARS-CoV-2, although, recent reports highlight adequate immune response and, notably, there haven’t been internationally acknowledged documented cases of reinfection.

Dr Nagarkar and team also plan to dig deeper in the future. In discussing the next steps, the researchers and policy makers behind the Pune serosurvey are considering going on a hunt for neutralising antibodies in the collected samples. This may give us more definitive insights.

Increasing antibody levels in the population is also being independently recorded by the private diagnostic centre and pathology laboratory Thyrocare Technologies Ltd.

The lab chain’s chief, Dr Arokiaswamy Velumani, says people are becoming increasingly immune with the daily rise in antibody levels.

“Immunity comes from antibodies. The prevalence of antibodies nationwide is a good number and every day, it is increasing. It is now at 26 per cent,” Dr Velumani told Swarajya.

This is a positive sign. Dr Velumani says the Covid-19 curve should begin to plateau after antibodies are detected in over 25 per cent of the population and should start falling when it reaches 35-40 per cent.

He also makes the case that herd immunity should not be looked at as a fixed threshold far out in the distance, but rather like a spectrum.

The more the antibody levels rise, the more people acquire collective immunity.

The 60-70 per cent threshold for herd immunity is not absolute. Arrived at as part of calculations to determine the extent of vaccination necessary for a disease, hinging on what's called the reproductive number, the immunity threshold could in reality vary across groups and regions.

Dr Velumani testifies to this variation from pincode to pincode after having carried out tests in over 2,000 pincodes in the country.

A recent New York Times report, too, suggests a lower herd immunity threshold, at 50 per cent, after speaking to more than a dozen top scientists.

Not to forget, antibodies aren’t even the full story of our body’s adaptive immune system.

T cells are being detected in people infected with the novel coronavirus.

Some people who have caught the infection have had T cells come into battle and offer protection on the basis of their past encounters with other coronaviruses.

Memory T and B cell responses, therefore, would be important to watch closely (how T cells work).

High prevalence in Pune

Is there an explanation for the high prevalence in Pune, especially in comparison to Delhi and Mumbai?

To some extent, the higher level makes sense as it can be put down to the periods when the samples were collected for testing — the Delhi and Mumbai surveys were conducted earlier in India’s Covid-19 journey (Both Delhi and Mumbai are doing their second rounds now).

But there is more to it than just timing. After all, the implication from the early data is that more than half of Pune may be infected.

Dr Nagarkar conjectures that infections may have risen predominantly in the months of April, May, and June when some people started going back to work out of compulsion and some others returned to their hometowns for the long haul.

“That was the period when testing was still not up to the mark, and infections may have been spreading in the community.”

In the Pune study involving five administrative zones, Lohiya Nagar-Kasewadi recorded the highest seropositivity of 65 per cent, with Kasbapeth-Somwarpeth coming in at the lower end with 36 per cent.

Yerwada, Rastapeth-Raviwarpeth, and Navipeth-Parvati were the three other prabhags identified for the survey. A total of 1,664 people were tested in these five areas, samples for which were collected in the period from 20 July to 5 August.

Notably, the size of the Pune survey was small. At 1,664 persons tested, the sample size was significantly lower than Delhi (21,387), Ahmedabad (30,054), and Mumbai (6,936).

Dr Nagarkar explained that the team had proposed a sample size of 4,560 to the Pune Municipal Corporation. The volunteers were to be picked from three categories of areas – high, medium, and low incidence, according to coronavirus case numbers – split evenly across the sample size. That is, 1,520 people from each type of area.

Experts whom the team consulted in the planning stages, however, advised to begin testing in high-incidence wards. The team subsequently narrowed down on areas where the virus spread was prevalent, and settled at one-third of the planned figure with a 10 per cent top-up, yielding a sample size of 1,664 people.

Does that mean the survey will extend to medium- and low-incidence wards?

Dr Nagarkar said with a smile, “It would be hard to locate low-incidence wards in Pune now,” referring to the infection upswing in Pune in recent weeks.

The coronavirus case tally in Pune stands at over 1.3 lakh people (18 August).

Mumbai was for a long time Maharashtra’s biggest worry, but Pune has surpassed the state capital in the number of cases.

After the recent surge, Pune is now second only to Delhi in that regard and is adding cases every day at a rate higher than both India’s political and financial capitals.

The sharp rise is being attributed mostly to the increase in testing — at 11,000, Pune’s daily average tests is a few thousand more than Mumbai.

Though, with active cases of more than 41,000, questions will be asked about Pune’s response to the pandemic.

In light of that, data from this and future seroprevalence surveys should guide Pune in devising a Covid-19 combat policy for the future.

As for the herd immunity question, it's still hard to say. But better answers may not be too far away.

Read the Pune report here.