Experts say ‘herd immunity’ could conquer COVID-19. But is it even possible?

Lively debate — and key unknowns — on how many people need to be vaccinated

People waited in line to get their vaccination in Danvers.Jonathan Wiggs/Globe Staff

For months, the two words have been a beacon for a weary world, touted by public health experts and government officials: If we reach “herd immunity,” they say, we can stomp out the coronavirus, emerge safely from the pandemic, and return to normal life.

The idea is a simple one: If enough people are vaccinated or otherwise become immune to the virus, it will run out of bodies to infect and eventually smolder out.

But as the nationwide vaccine drive kicks into high gear, the concept of herd immunity is fraying under the scrutiny. Far from a clear target that we can aim for in vaccination programs, herd immunity has become a catch-all term that may mean different things to different people — and it’s not clear whether using the strategy to fully squelch the virus is even possible.

Public health specialists offer a wide range of estimates when suggesting the target number of people who must be protected from COVID-19 for herd immunity to stamp out the virus, ranging from 60 to 90 percent of the population. Massachusetts has set a goal of vaccinating about 75 percent of adults in the state — 4.1 million residents — but that’s only roughly 60 percent of its entire population, including children. It’s an ambitious goal, but no one is sure if it will be enough to reach the threshold.

The reason for all the uncertainty? There is still a lot that we don’t know about both the virus and the vaccines.

Herd immunity itself is quite real, and we’ve seen it in action: Measles largely disappeared from the United States when about 95 percent of the population was immunized, according to the World Health Organization. Epidemiologists even have a formula to calculate how many people must be vaccinated to achieve herd immunity against any particular virus.

“It’s not hard to calculate if you knew all the numbers to put in,” said Dr. Marc Lipsitch, director of the Center for Communicable Disease Dynamics at the Harvard T.H. Chan School of Public Health. “What’s hard is to be sure about what numbers do go into it.”

In interviews, public health experts explained why the concept is so tricky. But they also stressed that immunizing a high portion of the population is still crucial, even if it won’t fully eradicate the virus.

An uncertain target

Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, has suggested that 70 to 85 percent of the population should be vaccinated to reach full herd immunity against COVID-19.

The considerable gulf owes to a yearlong struggle to fully understand a key characteristic of COVID-19: just how infectious the disease is when people aren’t taking precautions such as wearing masks or social distancing.

The world never got a clear look at that number, Lipsitch said.

Early on in the pandemic, many scientists came to believe that with COVID-19, each infectious person would infect 2.5 to 3 others, he said. If the number is 3, it would mean that reaching herd immunity would require about two-thirds of people to become inoculated, according to the mathematical formula.

But scientists may have underestimated the brutal efficiency of COVID-19 because early testing was so limited that it may have missed lots of people who would have tested positive. And it has since become impossible to get a more precise number, because so many people have changed their behavior.

“We we were sort of stuck with bad data,” Lipsitch said. “And there’s no really good way to get past it, because, appropriately, we weren’t spending our time trying to measure the number. We were spending time trying to make it lower.”

The rise of coronavirus variants also complicates this picture, because they are widely understood to be more infectious than the original form of the virus.

The higher the infectiousness rate, the higher the herd immunity threshold. If, for example, each person could infect four people, three quarters would need to be immune to make the virus dwindle. Five people would require 80 percent, and six would require nearly 85 percent.

“The threshold to achieve herd immunity is always a calculated back of the envelope,” said Dr. Virginia Pitzer, an epidemiology professor at the Yale School of Public Health.

Water and fire

Further muddling the math is another uncertainty: Whether — and to what extent — vaccinated people can still transmit the disease to others, potentially harming those who have not been immunized.

“If you think of people as tinder and vaccine as water to try to stop a fire, we don’t know how wet each person gets from a single vaccine,” Lipsitch said. “Whether they get so wet that they can never catch on fire, or if they just get wet enough that it’s harder to catch them on fire and they don’t burn as hot.”

Scientists are still exploring this question, and it could take some time to get a solid answer. Some research suggests the vaccines — already shown to dramatically reduce illness, hospitalization, and death — can also significantly reduce transmission, but maybe not entirely. Rochelle Walensky, director of the Centers for Disease Control and Prevention, said this week that the data so far is very promising.

Lipsitch suspects vaccines can reduce disease transmission by up to 80 percent — which he called “pretty good.” But if vaccinated people can still transmit the disease at some level, it will further increase the threshold needed to stamp out the virus.

“It’s a pretty good number, but pretty good does make things harder,” Lipsitch said. If the vaccine only cuts out about 80 percent of virus transmission, the real herd immunity target may be “pretty close to 100 percent” of the population vaccinated.

Is the target even realistic?

If eradicating COVID-19 will require population-wide immunization, it simply may not be feasible.

Even if the threshold is lower, significant numbers of people will likely not schedule appointments. And the vaccines haven’t yet been approved for those under 16.

“I think it’s going to be nearly impossible to achieve the herd immunity threshold, particularly with kids not being vaccinated,” Pitzer said. “If we were to immunize every age-eligible individual in the US, we wouldn’t achieve herd immunity.”

One additional factor may help, however: the millions of people who have already been infected by COVID-19, bringing some additional level of immunity to the population. While many will probably also get vaccinated, some could ultimately add to the legions of the inoculated.

“That will be our friend in the short run,” Lipsitch said. “It won’t last forever, because the more we reduce transmission, the more people will be born with no immunity or people’s immunity will wane. So it doesn’t solve the problem in the long run. But in the short run, it helps a lot.”

Strength in numbers

Lipsitch and Pitzer both stressed that despite these hurdles, it remains highly important to vaccinate large portions of the population.

Though the virus may not fully die out, a high uptake of vaccines would still substantially slow the spread — enough to potentially resume normal life. And if the virus will still circulate, protecting those most vulnerable would become especially critical.

“If we can turn this into a disease that is harmful, but to a smaller number of people, and on a smaller scale in terms of the hospital system, we will then decide as a society, as we do with the flu . . . to try to reduce it, but not to disrupt life on the same scale we’ve disrupted life so far to control it,” Lipsitch said. “We should not despair.”

Clarification: An earlier version of this story misstated what percentage of the population would need to be immune based on the infectiousness of a virus.