Q&A: What is herd immunity and what will happen to SARS-CoV-2 if we reach it?
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In recent comments regarding COVID-19, National Institute of Allergy and Infectious Diseases Director and chief presidential medical advisor Anthony S. Fauci, MD, referred to the concept of herd immunity as “an elusive number.”
Amid concerns of a potential fourth wave of COVID-19, we spoke with Lauren Ancel Meyers, PhD, director of the University of Texas at Austin COVID-19 Modeling Consortium, about the misconceptions regarding herd immunity and what may be necessary to reach the threshold.
Healio: What does it mean to reach herd immunity?
Meyers: There has been a lot of misinformation and misunderstanding about the herd immunity threshold (HIT). The important thing to understand is that when somebody gets vaccinated, they not only protect themselves from infection, but they also protect those around them. As more and more people in a community vaccinate, the virus will have a harder and harder time spreading. Because COVID-19 is such a stealthy virus — it spreads quickly and silently — it won't start to fade away until the vast majority of the people are immunized. And we won't get to this point as long as there are pockets of people who have low immunity levels. For example, children younger than 16 cannot yet get vaccinated, we have seen socioeconomic disparities in vaccine uptake, and there are religious and cultural subgroups with high levels of vaccine hesitancy throughout Texas and other parts of the United States.
Technically speaking, the HIT is the fraction of the population that has to be immunized before the virus' reproduction number drops below 1. But the specific value of the HIT is context-dependent. It depends on the efficacy and duration of immunity acquired through infection or vaccination, whether we have pockets of low immunity in our communities and whether there are emerging variants that can evade immunity.
Looking back at the fall of 2020, could we, as scientists and medical professionals, have done more to educate and correct false and misleading messaging about herd immunity? In October of 2020, the Great Barrington Declaration advocated for a “herd immunity” strategy in which we would essentially let the virus “rip,” at least through the young, healthy population. The White House embraced this strategy. It became a political mantra and even an article of faith for some.
Of course, building up herd immunity in a population through vaccination is far better than building it up through infection. The problem with infection-acquired herd immunity is that many people will end up in hospitals and die along the way.
Think about the 4 months that followed the Great Barrington Declaration, between last October and January of this year. We already knew so much about the virus and how to stop it, yet 270,000 people died from COVID-19 in the U.S.
Healio: What happens to the virus after a population reaches herd immunity?
Meyers: Herd immunity is complicated and is influenced by many different factors. We don't know if and when we'll get to the point that the virus is completely eliminated, particularly given complications like emerging variants and pockets of low vaccination coverage. Still, the closer we get to herd immunity by closing gaps in vaccine coverage and overcoming vaccine hesitancy, the safer, healthier and more open our society will be.
Healio: When could the U.S. be said to reach herd immunity against SARS-CoV-2?
Meyers: If we get to a point where the vast majority of our communities are vaccinated, there are no pockets of low immunity and the virus is no longer spreading, even when we lift measures like face masking and social distancing, then we can feel confident that we have achieved herd immunity. That said, herd immunity can be transient. If other parts of the globe haven't reached herd immunity, variants that evade immunity can emerge and once again threaten our cities.
Healio: Can mitigation efforts be relaxed if that happens?
Meyers: If we truly achieve herd immunity, then we should be able to relax mitigation measures, at least locally and temporarily. However, we will have to monitor the situation vigilantly, tamp out new outbreaks as they emerge and focus considerable efforts on eradicating the risk globally.
Even if we don't achieve full herd immunity, vaccines may help us get to a place where COVID-19 is a significantly less-lethal threat. As we reach high levels of vaccine coverage overall, especially in vulnerable populations, the hospitalization and mortality rates from COVID-19 will fall. If and when COVID-19 stops filling our hospitals and morgues, we may feel comfortable, relaxing mitigation measures, even if the virus is still spreading at low levels.
But I suggest we ask a slightly different question: “Can we increase some mitigation measures to accelerate our path to herd immunity?”
The HIT is the fraction of people who need to be immunized to stop the virus from spreading. Researchers have estimated that this may be 70%, 80%, or even higher, and that we can achieve herd immunity only if we reach these high levels of immunity everywhere. If there are pockets of low immunity — for example, in children, disadvantaged communities or subgroups with high levels of vaccine hesitancy — then we won't be able to stop the virus.
But we can lower the herd immunity threshold by maintaining low-cost measures like wearing a mask or ramping up other strategies to slow transmission. The United Kingdom’s new twice-per-week proactive testing initiative is one example. If we ramp up proactive testing programs to identify and isolate asymptomatic cases — in schools, colleges, workplaces, events, etc. — we can reduce transmission and get to herd immunity sooner.
Healio: In general, do you think the concept of herd immunity is well understood?
Meyers: At its core, the herd immunity concept is simple. Immunity provides both direct protection to the immunized individuals and indirect protection to the people around them. If enough people gain immunity, a virus will stop spreading, even though some people are still susceptible.
But in the real world, it's complicated. With emerging variants and pockets of low vaccination coverage, there is no guarantee we'll get there.
Nonetheless, taking the time to understand herd immunity and the factors that influence it can provide insights that will get us to a safer, healthier and more equitable and open society in the months ahead.
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