Are serological surveys false positives?

A serological survey means the collection of blood samples of individuals, done randomly to know the prevalence of Immunoglobulin-G (IgG) antibodies to fight Covid-19 and the extent of exposure to infection in the community.

In the last three months or so, such surveys have been conducted in some big cities/towns across States/UTs by different agencies. The notable ones are Delhi, Mumbai and Pune among cities and Bihar, Chhattisgarh, Odisha, Kerala, Telangana and Jharkhand among States.

In some places, only one such survey has been done, while in Delhi and Mumbai, two such studies have been done and more may be conducted, too.

The results showed high antibody positivity in cities such as Delhi, Mumbai and Pune, and low levels in rural States such as Bihar, Jharkhand and Chhattisgarh. The findings offer some good news for cities because a high positivity rate signals lower risk of Covid-19 while the low positivity rate in the hinterland shows that the possibility of coronavirus spread is still high.

To explain the point, the National Centre for Disease Control, in its first survey in Delhi during June 27 to July 10 (21,387 people across all 11 districts) revealed that about 23 per cent of the people have antibodies for the virus. In the second survey (in the first week of August) with a sample size of nearly 1,500, the positivity rate had improved to about 29 per cent.

Similarly, in Mumbai, the BrihanMumbai Municipal Corporation, in collaboration with NITI Aayog and Tata Institute of Fundamental Research, conducted a survey on July 3 in three wards with a sample size of just under 7,000. This survey revealed that about 57 per cent from slums and 16 per cent in other residential areas in the three wards — overall about 40 per cent — were exposed to the virus and had recovered silently. In Pune, the first survey done in five administrative zones (sample size 1,664) also showed high exposure rates, with 56-62 per cent among those living in slums and 44 per cent for living in bungalows/apartments.

Many grey areas

These results are being interpreted as an indication that the cities have hit their Covid-19 peak, and the fear of an explosion of infections is low.

The question now is whether these findings can be accepted by administrators at face value. No, for the findings suffer from the following lacunae:

First, it is not clear from the surveys if, and how, adjustments for false positives/negative were made. According to an advisory issued by the United States Centre for Disease Control on June 30, a positive antibody may also mean an infection like the one that causes the common cold.

Second, whether there can be re-infection among those who have recovered and how long will their immunity last even if the infection was mild. Third, most importantly, whether the herd (community) immunity, which is thought to have developed, will ensure protection against a further spike in the coronavirus disease.

Herd immunity is an epidemiological phenomenon in which a disease such as Covid-19 stops spreading in the population if it does not find adequate number of vulnerable people. Experts say that herd immunity is achieved when 40-65 per cent of the population develop antibodies.

However, with the number of cases rising continuously, there is a worry that infection could rise again . The number of cases, all India, basis, crossing 90,000, has surpassed even the US peak (just under 70,000), which is alarming. Is herd immunity really working?

Fourth, recovered Covid patients are reporting side-effects such as lethargy, disturbed sleep, body-ache, loss of appetite and even fever, respiratory and heart-related problems, liver and ophthalmological complications.

WHO chief Tedros Ghebreyerus recently said that the effects of the coronavirus would be felt for decades and lead to’ fatigue response’. While there are no peer-reviewed datasets, a survey of 1,657 people, called Survivors Corps, did show long-term effects. The UK’s Scientific Advisory Group for Emergencies fears that pathogens could be present forever in some form or the other. SAGE also said that Covid-19 will not be a disease such as small-pox, which could be eradicated through vaccination but, instead, it will be a “bit-like-flu” and people would need “re-vaccination at regular intervals”.

Far and wide

By all indications, Covid-19 is now moving to rural areas. In view of this, the ICMR, which recently started its second all-India serosurvey, should focus more on Tier-2/3 towns as also the villages. It must be realised that large countries like India cannot bank on herd immunity.

If at all, it can be achieved only through vaccination. And, according to a study published in the American Journal of Preventive Medicine, the vaccine has to be administered to at least 50 per cent of the population to break the chain of infection.

The writer is a former ISS