Covid cases plunge in the US – could herd immunity be at play?

Covid-19 is in retreat across the US. Cases have plunged by 77 per cent in six weeks and hospitalisations have dropped by almost 50 per cent in the past month.

The US vaccination programme is going well – 15 per cent of Americans have now received their first dose. But vaccines alone cannot explain this dramatic shift. Build-up of natural immunity may provide a more plausible explanation.

Covid cases have been rapidly and consistently declining since 8 January, at a time when almost nobody outside the healthcare industry had been vaccinated. Moreover, vaccines take weeks to kick in.

Vaccination is likely to be part of the reason why the decline in hospitalisations has accelerated – and why it’s likely to continue.

But to understand why the plunge started, we need to consider natural immunity. In other words, the virus may have had a harder time finding new bodies to infect.

“As more people have been infected, most of whom have mild or no symptoms, there are fewer Americans left to be infected,” says Dr Marty Makary, a professor at the Johns Hopkins School of Medicine and Bloomberg School of Public Health.

Dr Makary is optimistic that the decline will be sustained. “At the current trajectory, I expect Covid will be mostly gone by April,” he says.

While figures are still inconclusive, reinfection is estimated to have occurred in less than 1 per cent of people. When it has, cases have been mild.

These findings have implications for countries across the globe – not least the hardest hit nations such as the UK.

In the US, the Centre for Disease Control estimates that around 25 per cent of adults have antibodies from a previous Covid infection. But some scientists believe that in the US, as elsewhere, we are underestimating the number of people who have already built up some natural immunity to the virus.

Dr. Makary believes that levels of natural immunity in the US are likely closer to 55 per cent, since antibody tests fail to detect immunity deriving from antigen-specific T-cells.

And researchers at Sweden’s Karolinska Institute have identified T-cell immunity in individuals who were exposed to infected family members but never developed symptoms themselves.

Some sceptics have pointed to the recent decline in the number of Americans taking tests as a more realistic explanation for falling cases in the US. Indeed, according to The Atlantic’s Covid Tracking Project, the country’s coronavirus testing rates have declined steadily from more than two million a day in mid-January to about 1.6 million a month later.

Yet we can safely say this theory is overly pessimistic since the share of regional daily tests that are coming back positive has declined even more than the number of cases themselves. And reduced testing rates fail to explain why hospitalisations are plummeting. Fewer tests are likely due to declining demand as fewer people fall ill with the virus.

Of course, nobody knows yet exactly how long immunity will last. But we do know that there is a growing population with at least some level of protection.

Many scientists and politicians are afraid to talk about herd immunity, as Dr. Makary acknowledges. The term has political overtones and there is a fear that it encourages complacency or de-incentivises people to get vaccinated.

But, he adds, “herd immunity is the inevitable result of viral spread and vaccination.”

The virus needs bodies in order to survive and replicate, and the number of welcome hosts is diminishing.

Natural immunity, growing vaccination numbers – and warming weather – will continue to cut the number of viable bodies that would allow the virus to thrive. There is reason to feel cautiously optimistic that cases will continue to nosedive.