Coronavirus: why antibody rates vary so widely from place to place

Two-thirds of people in some New York communities have Covid-19 antibodies that are likely to provide some level of immunity to the virus, new research suggests.

The newly published data, from one of the companies involved in the US city’s testing programme, has renewed speculation about the reasons for huge variations in infection rates between countries, regions, cities and neighbourhoods.

In the aptly named district of Corona, “a working-class neighbourhood in Queens, more than 68% of people tested positive for antibodies to the new coronavirus”, The New York Times reports. “But at a clinic in Cobble Hill, a mostly white and wealthy neighborhood in Brooklyn, only 13% of people tested positive.”

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How do the figures compare?

The worst-affected areas of New York “appear to show a higher antibody rate than anywhere in the world”, based on the new data, says The Telegraph.

However, the tests were not carried out on a random sample of the population but rather volunteers, who may have stepped forward precisely because they believed they had been exposed to the new coronavirus.

Nevertheless, parts of Italy have recorded similar results.

“More than half the residents tested in Italy’s northern province of Bergamo have Covid-19 antibodies,” Reuters reported in early June. The results were based on a random sample that researchers believe was “a reliable indicator of how many people had been infected in the province”.

In Spain, by contrast, a study conducted at around the same time found that just 5.2% of people had the antibodies. And nationwide testing in India discovered antibodies in only 0.73% of the population.

Meanwhile, a random testing programme by the UK Office for National Statistics found antibodies in 17.5% of Londoners. The figure fell to 10% in the east of England, 12% in the northwest, and between 4% and 8% in the rest of the country.

What lies behind the variations?

The main reason that antibodies are distributed so unequally is that the coronavirus pandemic has struck some areas far more acutely than others.

In Italy, for example, the north and east of the country suffered one of the world’s worst outbreaks, while the southern half of the country was largely unaffected.

Chance may have played a part in where the infection took hold, but how quickly the outbreaks spread is likely to have been influenced by living conditions, transport networks and government policies.

“There are reasons parts of Queens were hit so hard,” says The New York Times. “Homes in Elmhurst and parts of Corona are especially crowded,” which led to higher rates of transmission among family members, a “leading driver of the disease’s spread”.

What does this mean for herd immunity?

Since the beginning of the global pandemic, experts have voiced scepticism about hopes of ending the pandemic by achieving herd immunity. Proponents of the approach claim that once between 60% and 70% of a population has had the new coronavirus, the infection will no longer be able to spread.

But while “previous research into other coronaviruses has indicated that antibodies confer immunity”, scientists have not established beyond doubt that the same applies with Covid-19, says CNBC.

“The accuracy of the antibody tests is not fully known,” adds The New York Times. “Nor is the extent of immunity conferred by antibodies or how long that immunity lasts.”

If antibodies are short-lived or offer less-than-total protection, testing for their presence may lead researchers to overstate levels of immunity in the worst-hit areas.

However, some scientists believe antibody tests alone may underestimate how many people have at least partial protection against Covid-19.

Studies in Sweden and Germany have found that large numbers of people with no antibodies specific to the new coronavirus still have some level of immune response, perhaps as a result of their bodies having previously tackled other types of infections.

Trials are also under way to determine whether the BCG vaccination may prime the immune system to fight off the virus currently sweeping across the world.