There was talk of immunity passports as early as late March, when the Sars-Cov2 virus which causes the coronavirus disease was raging through Europe (Dispatch 18, on April 5, wrote about them). Back then, the thinking was that these passports would be issued on the basis of antibody tests — with anyone with antibodies for Covid-19 being eligible for one. These people, the argument went, could move around, work, travel, and help cities and countries get back on their feet because they couldn’t get infected.

The idea was never implemented because researchers could not answer how long the antibodies or immunity (the two aren’t necessarily the same, but we will come to that shortly) would last. A recent study by researchers at the David Geffen School of Medicine, Los Angeles, may have the answer — Covid-19 antibodies have a half-life of 73 days, lower than the two years that antibodies for Sars-Cov-1 (which causes the Severe Acute Respiratory Syndrome) last. The two numbers are not directly comparable, but the difference is clearly there and significant. The study covered only 34 patients (and all standard caveats that apply to research based on a sample that small, do). The researchers also did not study what happens to the antibodies after 90 days, but admit that there is a chance the “decay will decelerate”.

The study’s finding — published in the New England Journal of Medicine — isn’t very different from another recent study published in pre-print server medRxiv by researchers from Kings College, London that found that Sars-Cov2 antibodies peak three weeks after infection, and then decline, with levels almost approaching the baseline in some cases in as little as two months. This sample size for this study was around 90 people including 64 patients.

But these studies do not necessarily mean any immunity we develop to Covid-19 is transient.

That’s because of T-cells (on which, suddenly, everyone seems to be an expert).

According to recent research at the Karolinska Institute and the Karolinska University Hospital in Sweden, twice as many people who have antibodies that fight Covid-19, have T-cells that target cells infected by the disease — which means the actual immunity in a population is higher than that measured by antibody tests. The researchers called this T-cell mediated immunity.

If true (the study sampled around 200 people), this would mean that in Delhi, for instance, where a sero survey (blood test) found that 22.86% of a randomly selected sample of around 21,000 people had antibodies, the actual level of immunity could be much higher, perhaps as much as 45%, a little over the 43% a study by the universities of Stockholm and Nottingham came up with as the herd immunity baseline for Covid-19. That could explain a lot of things — including why the number of daily cases started declining in the Capital.

This column wondered sometime back whether there was a peak infection rate after which the spread of the disease started declining. If the Swedish study holds, it could mean that the prevalence or infection rate itself is perhaps much higher than what is measured using antibody tests.

Indeed, T-cells are emerging as the real heroes of the pandemic. According to another recent study by Duke-National University of Singapore Medical School, Singapore’s National Centre for Infectious Diseases, and others, Sars-Cov2 generates specific T-cells and, more importantly, coronaviruses such as this one activate T-cells with long-lasting memory. For instance, the study found that people who were infected by Sars in 2003, and recovered, still have T-cells specific to Sars-Cov1, and that these T-cells actually help protect them from Sars-Cov2. The researchers said that this cross immunity, as they termed it, could explain why some people are either immune already, or react to the infection differently from others (for instance, they might not infect others, or the intensity of infection could be lower).

In essence, if all these studies hold, while the antibodies specific to Covid-19 might not last long, the number of people who are immune is actually higher than the number of those who have antibodies because of T-cells, which, it emerges, continue to be produced long after the infection in the case of coronaviruses.

I would treat that as really good news.