Question: I was reading an opinion piece in the Wall Street Journal that we could reach herd immunity by April.  Do you think that’s realistic?

Answer: The author, a medical doctor and professor at Johns Hopkins, argues that we’re witnessing this big drop in daily cases because so many people have already been infected; because we are quickly approaching herd immunity.  I have doubts about this assessment, which I’ve synthesized below.  For a refresh on the concept of herd immunity, check out our Q&A of 5/1.  And to examine different herd immunity scenarios, check out this article and set of data visualizations the New York Times published yesterday, When Could the United States Reach Herd Immunity? It’s Complicated. My hope is that we will continue to ramp up vaccine distribution and administration and my current optimistic expectation is that we will achieve herd immunity by the end of the summer.

1. The author doesn’t seem to take into account human behavior.  As cases spiked earlier last month and details emerged about new virus variants, we would have expected people to modify their behavior to reduce their risk, which in turn would have reduced the scope of community transmission.  More people wearing masks, more people double masking, fewer people traveling, fewer people attending large gatherings– these changes in behavior can have sweeping changes in the trajectory of the pandemic.  For example, let’s turn to our experience of previous waves. Remember the first terrible wave that hit New York (Figure 1)? Cases, hospitalizations, and deaths dramatically declined after mid-April, but the state was nowhere near herd immunity.  Indeed, serology testing soon after the first wave in New York City (a big hotspot) revealed that only 21% of city residents had been infected.  The end of the first wave was not due to herd immunity, it was because people and communities dramatically changed their behavior and New York was able to curb community transmission for many months.
2. The author rests his hypothesis on a mathematical guess that cannot be tested. The author posits that 55% of us have been infected with the virus, estimating that the ~28 million confirmed cases in the country represent only 10%-25% of all cases because of both asymptomatic transmission and limited testing.  He then goes on to describe why antibody testing cannot capture the full extent of all cases.  It is true that testing has not captured all cases and antibody tests are limited, but to make such a sweeping guess that 55% of us have already been infected without having solid evidence to back it up is suspect.
3. The author lets other hypotheses go unaddressed: In addition to the human behavior element I described above, other experts suggest the declining case rates could be due to increasing vaccination rates, to the changing seasons, or even to undertesting.   

Figure 1. Trends in Covid Cases, Hospitalizations, Deaths in New York State (data from covidtracking.com)