There are reasons to think the novel coronavirus began spreading earlier than previously understood, raising the possibility that herd immunity is closer than we think.
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The Icahn School of Medicine at Mount Sinai recently performed Covid-19 antibody tests on blood samples taken from New York City patients in February and March. They found 1.4% to 3.2% of emergency-room patients… tested positive for antibodies. Since antibodies can take a few weeks to develop, that suggests some New Yorkers were already infected by early February or even late January.
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Recent studies have also found that many people with mild or no symptoms who test positive for Covid-19 later don’t show antibodies when tested. Patients with mild symptoms produce a weaker antibody response than those who get more severely ill… These people, however, have been found to have long-lasting, potent T-cells that can ward off future infection. A small study last month from France found that six of eight close family contacts of sick patients didn’t develop antibodies but did develop Covid-19-specific T-cells.
In short, antibody tests may significantly underestimate the number of people who have already been infected with Covid-19, especially if they had a milder strain. If so, it’s possible that some early hot spots, like New York City and northern Italy, already have a degree of herd immunity. The same may be true of other places soon.
