In Berkeley, California, more than three-quarters of residents over age 16 have gotten at least one dose of a COVID-19 vaccine. But the numbers vary widely by neighborhood: In one of the city’s wealthier zip codes, 100% have reportedly gotten vaccinated. (In fact, the city’s dashboard says that more than 100% have been vaccinated.) In another zip code, only about 40% have. But the city as a whole reported no new COVID-19 cases on Monday, May 23.
Could Berkeley, and some other cities in the U.S., be at or near the point of herd immunity for COVID-19, where so many people are immune that the disease stops spreading? Some experts say it’s possible—even though people are continuing to travel in and out of areas with much lower vaccination rates. “When you’re at the point of herd immunity, stressing the system and bringing new people in or having people mingle, cases still don’t go up,” says Monica Ghandi, professor of medicine at the University of California San Francisco. “Around you, so many people are immune. And the un-immune, like children, are protected by lower case rates. If someone comes in and they’re unvaccinated, they’re usually standing next to someone who’s immune–once you’ve gotten to a certain degree of immunity, it’s hard for people to bump into each other.”
She points to Israel as an example. Though the country has lagged in getting vaccines to Palestinians, a large percentage of Israeli citizens were quickly vaccinated. “They started encouraging tourism to Israel, and even as people came in, it didn’t perturb their low cases and lead to increased transmission,” she says. (Israel also requires that any travelers present a negative COVID-19 test before they arrive.)
Epidemiologists have debated what it might take to get to a level of immunity that makes it impossible for an outbreak, throwing out numbers anywhere between 60% to 85% of people vaccinated. Ghandi says that the statistics from Israel suggest that the threshold might be around 60% of the total population, including unvaccinated children. Right now, as a whole, around 163 million people in the U.S. have gotten at least one dose so far, or roughly half of the population. But several cities are moving faster.
These are the metro areas with the highest rates of vaccination now, ranked by the percentage of the population who have received at least one dose as of the morning of May 24:
- San Jose, California, metro area, 65.3%
- The Villages, Florida, metro area, 65.2%
- San Francisco, California, metro area, 64.6%
- Napa, California, metro area, 64.6%
- Ithaca, New York, metro area, 64.4%
- San Diego, California, metro area, 64.2%
- Portland, Maine, metro area, 63.2%
- Burlington, Vermont, metro area, 62.7%
- Santa Cruz, California, metro area, 62.5%
- Greater Boston, Massachusetts-New Hampshire, metro area, 62.4%
Some doctors believe that herd immunity isn’t achievable. The concept originally came from observing disease in cows (hence the term “herd,” though it’s also called community immunity). After a certain number of cows were infected with a disease and survived, the disease stopped spreading, but only “if you don’t bring new cows from outside the herd itself,” says Inci Yildirim, a vaccinologist and associate professor at Yale School of Medicine and Global Health. If a community reaches the threshold where COVID-19 stops spreading, “You may have a pause or reduction in the pace of transmission, but it is not sustainable,” she says. “You’ll have other populations than your own where there are infected individuals, and within your own population, you will still have susceptible people. And with the interaction between those two populations, you will always have a risk for the exposure for the susceptible individuals that are coming from highly vaccinated pockets.” She also doesn’t believe that it’s possible to get to COVID-19 herd immunity without vaccinating children.
But if the country—or some states or cities—don’t reach the specific threshold of herd immunity, it’s clear that the more people who are vaccinated, the more slowly the virus will be able to spread in those who aren’t. But if it’s possible to go further, community immunity could protect immunocompromised people who can’t get the vaccine or who may have a weaker response if they do get the vaccine. That’s why it’s critical for vaccination rates to keep growing.
We’ve reached the herd immunity threshold for other diseases in the past, at least for some period of time. Measles spreads much more easily than COVID-19, but the vaccine is very effective and long-lasting, and we’ve seen that when 95% of a population is vaccinated, the remaining 5% are protected. The smallpox vaccine is also so effective, and vaccination rates were so high, that the disease disappeared entirely in the U.S. by the middle of the 20th century. The efficacy of the COVID-19 vaccines also matters–while the mRNA vaccines are very effective, herd immunity might not be possible with some of the others. In Seychelles, the world’s most vaccinated country, where the government used China’s Sinopharm vaccine and then AstraZeneca’s, cases are now increasing.
In the U.S., most people have gotten the Pfizer-BioNTech or Moderna vaccines. “I’m really hopeful, because the vaccines are so effective,” says Ghandi. “They’re so effective that it gets you there sooner, and you don’t need to vaccinate the whole population to get there.” Even as new variants spread, she’s optimistic. The variants are a problem “only as they evade our vaccines, and there’s no evidence that B.1.617.1 [the variant spreading in India] will evade our vaccines. So even if they’re seen here, and they’re more transmissible, there are just not people who are susceptible to it.”