Hunting for Herd Immunity: Proving Elusive in New York
After being one of the hardest hit U.S. cities by COVID-19 in the spring, New York City has taken a cautious approach to reopening. The City has experienced three months of relatively low case rates, but earlier this week Mayor de Blasio and Governor Cuomo started ringing alarm bells about rising case rates and rates of positive tests in some neighborhoods.
Ongoing SARS-CoV-2 antibody surveys in the City indicate 20–30% of the population may have been infected. As a result, at least one study has identified New York City as area that may have already reached herd immunity. On the other hand, two more recent studies of the pandemic in other countries (one study of several middle-income and developing countries, and one of Manaus, Brazil) suggest herd immunity was only reached when 50% or more of the population had been infected.
The Mayor and Governor identified nine neighborhoods in New York City where positive test rates were high. The prevalence of antibodies in the population in these new hot-spots range from 21–44%. At the high end of that range is Borough Park, which has had the second highest percent of its population (among New York neighborhoods) test positive for antibodies.
To get a further sense of how these hot spots compared to other NYC neighborhoods, I compared the antibody prevalence in the population to the recent rate of positive PCR tests for all of the City zip codes (similar to what I did for Spain).
This shows that five of the nine identified hotspots have positive rates above 30% in antibody tests putting them all in the top quartile (eight of the nine hot spots are above the median for antibody positive rate).
These hotspots have emerged only in the last few weeks and it is too early to tell how the case trends may develop. If the models estimating that New York City may have reached herd immunity were accurate, however, eight of these hot-spot neighborhoods should locally have achieved herd immunity, even if activity had returned to pre-pandemic levels. Yet mobility data for Queen’s and Bronx counties (which include the hotspot neighborhoods) remain well below the pre-pandemic baseline, and restaurants only this week returned to any indoor service in the City (at 25% of capacity).
Hopefully, cases in these areas will quickly peak and begin to fall again without the City having to re-impose restrictions. But if they continue to rise in these hard-hit neighborhoods, that would not only further undermine the lower projections of herd immunity, but also begin to raise the question of how long immunity lasts after infection.
For transparency, I also plotted the antibody prevalence rates in the neighborhoods against the case rate per 100,000 population for the most recent week (ending September 24). The neighborhoods with the high positive rates on PCR tests do not yet stand out in terms of case rate. Presumably the public health officials identified the hot spot neighborhoods because they have found a high PCR positivity rate to be more indicative of potential rising trends than higher case rates. But this is also a question worth exploring more.