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Public health experts offer a wide range of estimates when suggesting the target number of people who need to be protected against COVID-19 for herd immunity to eradicate the virus, ranging 60 to 90 percent of the population. Massachusetts has set a goal of vaccinating about 75% of the state’s adults – 4.1 million people – but that only represents about 60% of its entire population, including children. It is an ambitious goal, but no one knows if it will be enough to reach the threshold.
The reason for all this uncertainty? There is still a lot we don’t know about both the virus and vaccines.
Herd immunity itself is very real, and we’ve seen it in action: Measles largely disappeared from the United States when about 95% of the population was vaccinated, according to the World Organization health. Epidemiologists even have a formula for calculating how many people need to be vaccinated to gain herd immunity to a particular virus.
“It is not difficult 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 Harvard TH Chan School of Public Health. “What is difficult is to be sure of the numbers that go into it.”
In interviews with The Globe, public health experts explained why the concept is so tricky. But they also stressed that vaccinating a large part of the population is still crucial, even if it will not eradicate the virus completely.
An uncertain target
Dr Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, suggested that 70 to 85 percent of the population should be vaccinated to achieve full herd immunity to COVID-19.
The considerable gulf is due to a year-long struggle to fully understand a key feature of COVID-19: how infectious the disease is when people do not take precautions such as wearing masks or social distancing.
The world has never had a clear view of that number, Lipsitch said.
At the start of the pandemic, many scientists came to believe that with COVID-19, each infectious person would infect another 2.5 to 3, he said. If the number is 3, that would mean that to achieve collective immunity, it would take about two-thirds of people to be vaccinated, according to the mathematical formula.
But scientists may have underestimated the brutal effectiveness of COVID-19 because early testing was so limited that it may have missed many people who would have tested positive. And it has since become impossible to get a more precise number, as so many people have changed their behavior to avoid contracting COVID-19.
“We were kind of stuck with bad data,” Lipsitch said. “And there’s no real good way to get past it because, appropriately, we weren’t spending our time trying to measure the number. We spent time trying to reduce it. “
The rise of coronavirus variants also complicates this picture, as they are widely recognized to be more infectious than the original form of the virus.
The higher the infectivity rate, the higher the herd’s immunity threshold. If, for example, each person could infect four people, three quarters would have to be immunized to decrease the virus. Five people would need 80% and six would need almost 85%.
“The threshold for achieving herd immunity is always a calculated back of the envelope,” said Dr. Virginia Pitzer, professor of epidemiology at the Yale School of Public Health.
Water and fire
Another uncertainty is whether or not to blur the math even further: whether – and to what extent – vaccinated people can still transmit the disease to others, which could harm those who have not been vaccinated.
“If you think of people like tinder and vaccines like water to try to stop a fire, we don’t know how wet every person is with the one vaccine,” Lipsitch said. “Whether they’re so wet that they can never catch fire, or whether they’re just wet enough that it’s harder to catch them on fire and they don’t burn as well.”
Scientists are still exploring this question, and it might take some time to get a solid answer. Some research suggests that vaccines – which have already been shown to dramatically reduce illness, hospitalizations and deaths – can also significantly reduce transmission, but perhaps not entirely. Rochelle Walensky, director of the U.S. Centers for Disease Control, said this week that the data to date is very promising.
Lipsitch suspects that vaccines can reduce disease transmission by up to 80% – which he called “very good”. But if vaccinated people can still transmit the disease to a certain level, it will further increase the threshold needed to eradicate the virus.
“That’s a pretty good number, but pretty good makes it more difficult,” Lipsitch said. If the vaccine eliminates only about 80 percent of the transmission of the virus, the true target of herd immunity could be “quite close to 100 percent” of the vaccinated population.
Is the target even realistic?
If eradicating COVID-19 will require population-wide vaccination, it simply won’t be feasible.
Even if the threshold is lower, a significant number of people will probably not make an appointment. And vaccines have yet to be approved for those under 16.
“I think it will be almost impossible to reach the threshold of herd immunity, especially with children who are not vaccinated,” said Pitzer. “If we were to vaccinate everyone eligible for age in the United States, we wouldn’t achieve herd immunity.”
An additional factor, however, can help: the millions of people who have already been infected with COVID-19, bringing an additional level of immunity to the population. While many of these people will likely be vaccinated as well, some may eventually add to the legions of people vaccinated.
“He will be our friend in the short term,” Lipsitch said. “It won’t last forever, because the more we reduce transmission, the more people will be born without immunity or people’s immunity will decrease. So that doesn’t solve the problem in the long run. But in the short term, it helps a lot.
Strength in numbers
Lipsitch and Pitzer both stressed that despite these obstacles, it remains very important to immunize a large part of the population.
Although the virus will not die out completely, heavy vaccine use would still significantly slow the spread – enough to potentially resume normal life. And if the virus continues to circulate, protecting those most vulnerable to the disease would become particularly critical.
“If we can turn that into a bad disease, but for a smaller number of people, and on a smaller scale in terms of the hospital system, then we as a society will decide, as we do with the flu. . . to try to reduce it, but not to disrupt life on the same scale that we have disrupted life so far to control it, ”Lipsitch said. “We must not despair.”
Adam Vaccaro can be reached at [email protected]. Follow him on Twitter at @adamtvaccaro.
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