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 [the herd immunity threshold] 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 with the Globe, 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 to avoid getting COVID-19.
“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…