The fourth round of antibody tests in New Delhi, results of which were made public on Wednesday in the course of a court hearing, presents some interesting questions.
It found that 25.5% of the approximately 15,000 people tested possessed Covid-19 antibodies. The tests were carried out in the third week of October. The results are near-identical to those of the third round, covering around 17,000 people, conducted in the first week of September, which showed that antibodies were present in 25.1% of the people surveyed. The second round (around 15,000 were tested in August) showed that 29.1% of the people surveyed had antibodies. And the first round, in late June and early July, found them in 22.6% of the 21,000 people surveyed.
This is not how the results of sero surveys are meant to progress. Sero surveys, such as the ones being carried out in Delhi, are a measure of prevalence of an infection (in this case, of Covid-19), and, therefore, of immunity. Ideally, their results should show increasing prevalence, or immunity, till the herd immunity level is reached. This is the level at which much of the population is safe because the virus can’t find enough people to infect. But there are complications in making such assumptions: for one, Covid-19 antibodies do not seem to last; for another, the presence of antibodies isn’t always necessary for immunity, just as their absence isn’t a sign of lack of protection. Indeed, the October round of the sero survey in Delhi found that 43.5% of people previously diagnosed with Covid-19 did not possess antibodies any longer.
This isn’t entirely unexpected.
In late October, results of a study led by Imperial College London, of 365,000 people across England, conducted in three rounds between June and September, showed that the number of people with Covid-19 antibodies declined over time. The UK study actually saw a decline in the infection’s prevalence across all parts of the country, with a clear fall between the first and the third rounds. The study was conducted ahead of the second wave of infections in the UK. This may well be what the Delhi study is finding too.
There is some debate about the behaviour of antibodies, though. According to a paper published in Science Immunology in early October by researchers in the US, including some in Massachusetts General Hospital, and based on a study of 343 Covid-19 patients, antibodies of the immunoglobulin G (IgG) variety, ones that typically provide long-lasting immunity, were found in patients for up to four months (the study ended then). Others, of the IgA and IgM variety, were detected 12 days after infection but did not last beyond two months.
A second paper, also published in the same journal around the same time, but based on a study in Canada by researchers at the University of Toronto, came to a similar conclusion – levels of antibodies of the IgG variety reached a high between two weeks to a month after the infection, and then remained stable for at least three months. This study covered 439 people (not all had been tested, but all did have symptoms of Covid-19).
Interestingly, as Dispatch 112 on July 23 pointed out (citing a late June paper on a preprint server), research at Sweden’s Karolinska Institute and Karolinska University Hospital showed that many infected people who were either asymptomatic or showed only mild symptoms did not test positive for antibodies although they had what the researchers called a strong T-cell response. The research was subsequently published in Cell. T-cells are the immune system’s combat specialists (even better, they remember viruses), recognising and fighting infected cells. The research (of around 200 people) showed that twice the number of people with Covid-19 antibodies had T-cell immunity. This could well mean that people who are asymptomatic or contract a mild infection either do not generate these antibodies or see their number wane soon after the infection passes, although (according to the Swedish study) they remain immune. Is that what Delhi’s result is showing? And what does that mean in terms of the infection’s prevalence and immunity?