How Far Are We From COVID-19 Herd Immunity?

A rough and optimistic projection for the pandemic ending sooner rather than later.

The COVID-19 vaccine rollout in the United States is picking up speed. So far, nearly 74 million Americans have received one dose and around 40 million have completed a full vaccination regimen. Herd immunity will be attained through a combination of vaccinations and infections. Herd immunity is achieved when there are so many vaccinated and formerly infected people that the virus has a hard time finding immunologically unprotected people to infect.

Some researchers had earlier suggested that the herd immunity threshold for the COVID-19 coronavirus would be relatively low, but more recent research finds that around 60 to 80 percent of the population will have to be immune to end the pandemic. For argument's sake, let's set the U.S. herd immunity threshold at 70 percent. How close are we to achieving herd immunity and post-pandemic normality?

Nearly 30 million Americans have so far been diagnosed with COVID-19. However, it is well-known that a huge number of infections go undiagnosed or are asymptomatic. Unfortunately, U.S. public health authorities have failed to establish a COVID-19 antibody blood test population surveillance system that could have provided this information. However, a couple recent reports of antibody blood test surveys offer hints as to just how extensive COVID-19 infections have been so far.

A new study in the Journal of the American Medical Association used the nationwide blood testing results of about 62,000 people who were applying for life insurance. On the basis of those data, the researchers estimated "that 15.9 million asymptomatic or undiagnosed SARS-CoV-2 infections had occurred in the United States as of September 30, 2020." At the time, the total number of diagnosed cases stood at just over 7.5 million.

Another new study by University of Texas researchers analyzed population and patient COVID-19 antibody results and preliminarily report that about 20 percent (5.8 million) of Texans have so far been infected with the coronavirus. Interestingly, in the population sample, 29 percent of Texans younger than 19 years of age have COVID-19 antibodies. Total diagnosed cases in the Lone Star State, meanwhile, are about 2.7 million.

The results of these recent serological surveys suggest that undetected infections are twice as high as diagnosed cases. Since some 30 million cases have been diagnosed, the serological surveys suggest that 90 million Americans have actually been infected with the virus so far. This infection rate is close to the 98 million estimate made by data scientist Youyang Gu in his path to normality projections.

So assuming herd immunity kicks in when 230 million Americans (70 percent) are either vaccinated or have been infected and recovered, how far are we from that goal? Let's just do a very rough and optimistic calculation by assuming significant immunity among the 74 million Americans who have already gotten at least one dose of vaccine, plus the infection number of 90 million. That adds up to 164 million people who currently have antibodies against the virus, which is about 70 percent of the way toward the goal of 230 million people.

At the current rate of 2.5 million vaccinations per day, another 75 million Americans will be partially or fully vaccinated in the next month, bringing the total number of people with antibodies to 240 million by the middle of April. It is worth noting that in Israel, where about half of the population has so far been inoculated, COVID-19 cases are plummeting.

Of course, these rough calculations are complicated by the fact that there is some overlap between vaccinated and previously infected people; one-dose versus two-dose vaccines; and the spread of more transmissible and deadly virus variants, which implies a higher overall herd immunity threshold.