A nationwide survey involving testing the blood of people to check for the presence of Covid-19 antibodies has turned up good news for those who’ve recovered from the coronavirus. Antibodies to protect against catching the virus again remain active for at least six months, the study by the Council of Scientific and Industrial Research (CSIR) suggests.

“Even after six months, antibodies and neutralising antibodies, although they decreased from the initial values, were much above the (protection) threshold in most of the cases,” says Shantanu Sengupta who’s the senior principal scientist at the CSIR-Institute of Genomics and Integrative Biology (IGIB) and one of the study’s co-authors.

The landmark study, the first of its kind to be conducted in India, also suggests that smokers and vegetarians enjoy a lower risk of catching the virus. Those with the O and A group blood types may be less likely, too, to get infected while those in the B and AB blood groups are at most risk from Covid, the report says.

The CSIR-Institute of Genomics and Integrative Biology, known better as IGIB, surveyed staff and their family members at the CSIR’s more than 40 laboratories and institutes in 24 cities across India. It found that 10.14 per cent of 10,427 individuals surveyed had antibodies to SARS-CoV-2, the virus which causes Covid-19.

“Use of private transport, lower (public) exposure occupations, smoking, vegetarianism, and ‘A’ or ‘O’ blood groups appeared to be protective (against Covid) using seropositivity as a surrogate for infection,” the report says.

Seropositivity simply means showing a positive result in a blood test for a particular antibody. Antibodies are substances in the blood that fight disease. Seroprevalence was highest for blood group type AB (10.19 per cent) followed by group B (9·94 per cent), group O (7.09 per cent) and the lowest for group A (6.97 per cent).

A follow-up survey carried out on 346 seropositive individuals after three months found “stable” to “higher” antibody levels against SARS-CoV-2, while there was a fall in Covid virus-neutralizing plasma activity.

A repeat check on 35 participants at six months showed declining antibody levels but the neutralising plasma antibodies remained steady compared with the level recorded three months earlier.

The crucial takeaway

Even so -- and this is the crucial takeaway -- both the normal antibody levels as well as the neutralizing plasma antibodies were still above the level required to fight off infection.

“Even after six months, antibodies and neutralising antibodies, although they decreased from the initial values, they were much above the (protection) threshold in most of the cases,” Sengupta says.

“This is an important finding that means people who are already infected and seropositive, their antibodies will remain intact at least for six months,” he says. The study found that 95 per cent of the seropositive individuals also had "significant plasma virus-neutralising activity."

“The antibodies are much higher than the threshold. That’s very heartening to know,” adds Sengupta. The institute plans to continue following the same people “for a long time so see how many are getting infected and how stable the antibodies are,” he says.

It’s the first time in India that researchers have tracked individuals for six months to check for Covid-19 antibodies. And the institute plans to keep on following the same people “for a long time so see how many are getting infected and how stable the antibodies are,” Sengupta says.

Socio-economic snapshot

The institute chose to study its staff because of the large number of employees and students that it could survey as well as its geographic reach with centres in 24 cities across India.

The staff and students represented a socio-economic snapshot of India. “The CSIR is present in all parts of the country and CSIR individuals represent the urban middle-class as well as contractual staff and the low socio-economic class. That’s why we say we captured the entire nation,” Sengupta says. “About 15-16 per cent of the people who were of lower socio-economic status were seropositive while for the others it was 5-6 per cent,” he notes.

The aggregate seropositivity of 10.14 per cent suggests that more than 100 million Indians were infected by last September, primarily in larger cities but the illness was rapidly spreading all over the nation at that time. Most seropositive individuals had occupational exposure risk such as being in public-facing jobs working as security guards. Significant symptoms were recalled by only 25 per cent of those surveyed, with 75 per cent recalling no symptoms at all.

No sight of herd immunity

At the same time, the paper says that “herd immunity does not seem to be in sight and an effective vaccine may be necessary to control this pandemic. Large cities have high seropositivity rates, “but the majority of India continues to have low-to-moderate seropositivity,” the researchers said.

The study also sought to determine whether seropositivity could be an indicator of future virus outbreaks. Researchers plotted on a graph the rate of new cases before and after the date of taking blood samples. The researchers found that “higher seropositivity predicted declining new cases, while the largest increases were seen exclusively in areas of low seropositivity.

This is “a very important message we wanted to give from this study. This way (through seropositivity) you’ll be actually able to predict which are the places which will have more prevalence of the disease,” Sengupta says.

Smoking may be protective, but…

The study’s observation that smokers are less likely to be Covid seropositive is the first such finding involving the Indian population. Non-smokers recorded a higher seroprevalence of 10.11 per cent, as opposed to smokers (6.88 per cent). The finding contributes to a growing body of evidence which suggests that despite COVID being a respiratory disease, smoking may be protective. Two studies from France and similar reports from Italy, New York and China have also found lower Covid infection rates among smokers.

“That smokers have a lower seropositivity rate has been found in various countries but they were all hospital-based studies. This is the first population study to suggest that,” says Sengupta. “But in no way are we supporting the idea that smoking is good,” he adds, laughing.

The French study carried out at the Pitie-Salpetriere hospital suggested that a substance in tobacco – possibly nicotine – might be stopping patients who smoke from catching Covid-19. Clinical trials of nicotine patches are awaiting the approval of the country’s health authorities.

However, the French researchers said that while nicotine may protect smokers from the virus, those who have caught Covid often develop more serious symptoms because of the harmful effect of tobacco smoke on the lungs. The Italian study showed that smokers were five times less likely to end up in the hospital with Covid but -- and this is key -- nearly twice as likely to die if they were admitted.

Occupation and infection risk

Non-vegetarians, meanwhile, had a seroprevalence rate of 11 per cent, while seropositivity among vegetarians was 6·86 per cent. One possible reason scientists put forward for the lower infection rate among vegetarians is that they consume a plant-based fibre-rich diet that promotes gut health which in turn can enhance immunity.

Occupation also figures importantly in infection risk. The researchers clubbed the individuals surveyed into two broad categories: CSIR staff and so-called “outsourced staff.” The staff consisted of faculty, students, office staff and family members. The outsourced staff worked in such areas as security, housekeeping and grounds maintenance. The CSIR staff had a positivity of 7.98 per cent, while positivity for the outsourced staff, who have more interaction with the general public and often travel by public travel, was more than double that rate at 16.22 per cent.

Interestingly, the presence or absence of diabetes, hypertension and cardiovascular disease were not found to have a significant effect on seropositivity.

Large cities with high seropositivity rates

The highest virus positivity rate was found in Hyderabad at 18 per cent of the 2,073 people whose blood was tested for Covid antibodies (eight percentage points higher than the national virus positivity rate). The second-highest positivity rate was detected in Chennai at 16 per cent, followed by Delhi at 14 per cent.

Large cities have high seropositivity rates, but the majority of India continues to have low-to-moderate seropositivity, the study says. The study provides an important “snapshot across multiple geographies” and it confirms that by last September “there was a wide but uneven spread of the pandemic with the epidemic subsequently growing more rapidly in regions with low seropositivity,” the researchers conclude.

On the bright side, Covid cases now are “constantly falling,” says Sengupta, outlining two potential reasons. “It’s possible a lot of people have already been infected. Secondly, people have understood and are taking more precautions,” he says. "And if enough people have already obtained immunity, that’s definitely good.