STEVE INSKEEP, HOST:
The spread of vaccines has begun moving the United States closer to herd immunity - that's the point where so many people are immune to coronavirus that it fades. But if we are closer, we're not yet close. Scientists say between 70% and 85% of all people need to be immune. Here's NPR science correspondent Richard Harris.
RICHARD HARRIS, BYLINE: The term herd immunity has been tossed around a lot during this pandemic. It's worth remembering what it means exactly.
LAUREN ANCEL MEYERS: The idea of herd immunity is that after enough people have been infected or vaccinated, the virus will start to subside on its own because there's just not enough susceptible people left to infect.
HARRIS: Lauren Ancel Meyers at the University of Texas says it's not a magic moment after which the virus simply vanishes, but the disease should gradually ebb whenever we reach that point, whether it's 70%, 85% or some other percentage of the population.
ANCEL MEYERS: However, there's a lot of complexities with this pandemic that make herd immunity a little bit more of a complicated and possibly elusive concept.
HARRIS: New variants of the virus, for example, could create a new wave of infections, even in people who have been vaccinated or previously exposed. That could slow progress toward herd immunity. Another big question is what to make of the people who have already been infected. Researchers at Columbia University estimate the figure at more than 100 million - about 30% of the U.S. population. Dr. Kari Nadeau at Stanford University says they're giving us a head start toward herd immunity.
KARI NADEAU: I think you can say asymptomatic, mild, moderate, severe - if you've had COVID before, then you can be added to the herd immunity number.
HARRIS: But Dr. Stanley Perlman, who studies coronaviruses at the University of Iowa, isn't so sure.
STANLEY PERLMAN: People who have more severe disease are probably better protected from being reinfected. People who are vaccinated, of course, are going to be better protected from being infected. But what we don't know is the - somebody who had asymptomatic infection or very, very, very mild infection, how long will their immunity last?
HARRIS: And that's an important point because a large fraction of the 100 million estimated to have had previous infections may have had mild symptoms or none at all. Perlman points to studies of other coronaviruses, those that caused the mild symptoms of the common cold. People don't remain immune to them for all that long.
PERLMAN: We know they're protected on the average for about a year, maybe longer, sometimes shorter. But it's certainly not permanent protection.
HARRIS: So it could be a race between getting enough vaccine into people's arms before natural immunity wanes in people who were previously infected with the coronavirus. Dr. Nadeau agrees we simply don't know how long natural immunity will last.
NADEAU: And that's why for people that get COVID naturally, they should definitely get the COVID vaccine.
HARRIS: Even given the best-case scenario, if all 100 million Americans previously exposed to the virus are immune, it still won't be easy to achieve 70% to 85% immunity in the population. That figure would include children, who make up 22% of the U.S. population, and vaccines are not approved for anyone under 16 right now. Dr. Anthony Fauci at the National Institutes of Health says studies are ongoing to show whether vaccines are safe and effective in children.
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ANTHONY FAUCI: So that, hopefully by the time we get to the late spring and early summer, we will have children being able to be vaccinated, according to the FDA's guidance.
HARRIS: And one last challenge - many Americans say they don't want to get vaccinated. If enough people decline, herd immunity could remain out of reach. However, a significant number of people who don't want the vaccine may already have been infected, so that natural immunity could help close the gap, presuming that immunity persists for a while.
Richard Harris, NPR News.
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