Has New York City Achieved Covid19 Herd Immunity?

New York State Governor, Andrew Cuomo announced today that random surveys indicatd that 14% of New York State residents now have antibodies to Covid19, and in New York City, the number is 21%.

Assuming that being antibody positive means Covid19 immunity, that is immunity from re-infection, has New York achieved so-called herd immunity, which is to say immunity to the Covid19 virus that is sufficiently widespread to prevent each newly infected individual from, on average, passing the virus to more than one other person?

If herd immunity has been achieved, then the decline in reported deaths should continue even if the present measures of quarantine and lock-down are abandoned.

Unfortunately, there is no clear guide as to the infection frequency required for herd immunity. Assuming a reproduction number of more than three, which is to say the average number of people each infected person infects before the development of any population immunity, then an infection rate of over 60% of thereabouts would be needed to end the epidemic, assuming that people interact and spread the disease in a random fashion. However, people do not interact in a random fashion. Some are much more effective as disease spreaders than others. Children, for example, are excellent spreaders, both among themselves and among family members and family friends. Old folks, however, tend to live relatively isolated lives and are therefore extremely poor spreaders. It is not implausible, therefore, to suppose that an immunity rate of 21, as in New York City, or even as low as 14% for New York State as a whole, is sufficient to cause the epidemic to die.

As the various jurisdictions dealing with the epidemic make decision on ending lock-downs and other quarantine measures, and as more surveys of population immunity are reported, we will soon know.

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