Of all the moving goal posts in the COVID-19 pandemic, herd immunity may be the most difficult to pin down — and it may even be the wrong metric to track.
Herd immunity is the concept that once a majority of the population has been vaccinated or has natural immunity to an infectious disease, in this case SARS-CoV-2, life will shift back to normal.
But to listen to Dr. Anthony Fauci, President Joe Biden’s chief medical adviser and the head of the National Institute of Allergy and Infectious Diseases, is to hear him downplay the focus on this “elusive” concept and instead urge Americans to pay attention to other figures like vaccination rates.
“We should not get so fixated on this elusive number of herd immunity,” Fauci said March 15. “We should just be concerned about getting as many people vaccinated as quickly as we possibly can because herd immunity is still somewhat of an elusive number.”
Fauci has referred to herd immunity as an “elusive” concept at least seven times over the last two months during the White House’s COVID-19 briefings.
Here’s why the concept of herd immunity is so complicated:
- There is no standard percentage to get to herd immunity. Gavi, a global health organization focused on immunization, puts the benchmark for herd immunity at 60%. Fauci estimates that herd immunity will occur when between 70% and 85% of the population is protected. Even Biden has weighed in. “There’s a debate about what constitutes herd immunity,” he said Monday. “Is it 70%? 68%? 81%? The point is right now every single person 16 years or older doesn’t have to wait in line. Just show up and get a vaccination now.”
We don’t know how long immunity to the virus lasts, both in people who have recovered from COVID-19 and those who have been vaccinated. Moderna Inc.
recently said participants in the Phase 1 clinical trial still had antibodies six months after getting the second dose of its COVID-19 vaccine, while Pfizer Inc.
said its vaccine had a 91% efficacy rate in clinical-trial participants six months after they received the second shot. “Herd immunity is kind of an elusive terminology because we don’t know what that percentage of protection is,” Fauci said April 9. It “will be a combination of people who are protected by vaccination, as well as those who were infected, who have recovered, and now have protective immunity.”
- As we start to exit this phase of restricted living — working from home, wearing masks, and steering clear of large events that have all but been shuttered — behavioral changes may also impact the path to herd immunity. This could mean that looser social-distancing practices may increase people’s exposure to the virus. In addition, immunity to the virus could differ in the winter versus the summer, similar to how common colds tend to affect more people in the winter months in the Northern Hemisphere, according to researchers writing a JAMA viewpoint last month.
What appears to be a much clearer concept is how life could change if herd immunity is reached. In that futuristic world, we would throw away our masks, gather indoors with strangers, and push our way onto crowded trains and into packed concerts.
But if herd immunity is as difficult to track as Fauci says it is, then what is the best metric to track our way out of this pandemic? Is it vaccination rates? Is it the scientific R0 figure, which explains how contagious the virus is? What about the number of hospitalizations and deaths?
“We need to get away from waiting for this mystical elusive number and just … get as many people as we possibly can get vaccinated as quickly as possible,” Fauci said in April.
“We don’t want to get too hung up on reaching this endgame of herd immunity because every day that you put 2 million to 3 million vaccinations into people [it] makes society be more and more protected,” he noted a month earlier.
More than 105 million people in the U.S. are fully vaccinated, as of Tuesday, according to the Centers for Disease Control and Prevention. That’s essentially one-third of the U.S. population. That said, public-health experts are increasingly concerned that vaccination is going to slow down in the coming weeks, and immunization rates may plateau. The Kaiser Family Foundation predicts that by mid-May all the people in the U.S. who wanted a vaccine will have gotten their shots.
“Once this happens, efforts to encourage vaccination will become much harder, presenting a challenge to reaching the levels of herd immunity that are expected to be needed,” KFF’s health policy experts there wrote in April.
(One caveat is that if Pfizer’s vaccine is authorized within the week for 12-to-15 year-olds, as sources told the New York Times, that may boost vaccine interest for a group that hasn’t been able to get immunized yet.)
In a note to investors last week, RBC Capital Markets’ Brian Abrahams wrote that the “threshold for ‘herd immunity’ may be a moving target, but there are some interesting clues on when transmission may slow; increasingly look toward reductions in overall cases and morbidity/mortality to better guide potential return toward normalcy.”
Abrahams expects “dramatic drops” in new cases by June in the U.S. but also an increase in cases in the fall and winter. But reaching “true” herd immunity may be a wobblier line, based on the emergence of more infectious variants, waning immunity, and plateauing vaccination rates as a result of the vaccine hesitant.