Smokers and vegetarians were found to have lower sero-positivity, while those with blood group 'O' may be less susceptible to coronavirus, according to a survey by an top research body.
The pan-India serosurvey, conducted by the Council of Scientific and Industrial Research (CSIR), studied the presence of antibodies against SARS-CoV-2, the virus that causes the coronavirus disease (COVID-19), and their neutralisation capability to infer possible risk factors for infection.
The study, done by a team of 140 doctors and scientists, assessed 10,427 adult individuals working in more than 40 CSIR laboratories and centres in urban and semi-urban settings, and their family members. These people voluntarily participated in the study.
The survey suggested that despite COVID-19 being a respiratory disease, smoking might be acting as the first line of defence due to its role in increasing the mucous production.
It, however, cautioned that there was a need for focused mechanistic studies to understand the effect of smoking and nicotine on coronavirus infection.
"Smoking is known to be severely detrimental to health and associated with multiple diseases and this observation should not be taken to be an endorsement, especially given that the association is not proved to be causal," the paper stressed.
It indicated that vegetarian food rich in fiber might have a role to play in providing immunity against COVID-19 due to its anti-inflammatory properties by modification of gut microbiota.
The survey also found that those with blood group 'O' may be less susceptible to infection, while 'B' and 'AB' were at a higher risk.
Sero-positivity was highest for blood group type 'AB', followed by B, it said, adding that blood group 'O' was found to be associated with a lower positivity rate.
Shantanu Sengupta, one of the co-authors of the paper, said the paper was recently peer-reviewed.
Earlier, two studies from France and similar reports from Italy, China and New York reported lower COVID-19 infection rates among smokers.
A study by Centers for Disease Control and Prevention (CDC), which examined over 7,000 people in the US who tested positive for COVID-19, also had similar findings.
Interestingly, the study found that only 1.3 per cent of survey participants were smokers, compared to the CDC report that 14 per cent of all Americans smoke.
Similarly, UCL (University College London) academics that looked at 28 papers across the UK, China, France and the US, and found the proportions of smokers among hospital patients were lower than expected.
One of its studies showed that in the UK, the proportion of smokers among COVID-19 patients was just five per cent, a third of the national rate of 14.4 per cent.
Another found in France, the rate being four times lower -- 7.1 per cent vs 32 per cent among all populations.
In China, a study noted that only 3.8 per cent of patients were smokers despite more than half of the population regularly smoking cigarettes.
A separate study by Jin-jin Zhang to understand the influence of smoking behaviour on the susceptibility to coronavirus observed that only nine patients -- 6.4 per cent -- had a history of smoking and seven of them were past smokers.