Since the coronavirus pandemic swept the world in 2020, many people have anticipated what's known as herd immunity as the elusive finish line. One year ago, top Boston doctors couldn't say when, if ever, we'd cross it.
Now, it's possible the finish line is in sight.
Herd immunity relates to the idea that a high level of immunity to a virus in a population can be achieved when a lot of people form antibodies, both from fighting the virus through a natural infection and from vaccination. When herd immunity is reached, in theory, the virus has nowhere to go and will die out.
Estimates based on measles and other diseases suggest that herd immunity occurs when 70-80% of the population is vaccinated.
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Now that about 80% of the Massachusetts population is fully vaccinated and the majority of residents have contracted the virus, Tufts Medical Center's Dr. Shira Doron, Brigham and Women's Hospital's Dr. Daniel Kuritzkes and Boston Medical Center's Dr. David Hamer revisited the question during the latest episode of NBC10 Boston's weekly "COVID Q&A" series.
Between widespread uptake of COVID-19 vaccines and natural infection, the U.S. has developed a "very high level of immunity," according to Kuritzkes, Brigham and Women's chief of infectious disease.
"People have both natural and vaccination-induced immunity and that is what is protecting everybody," Kuritzkes said. "So, in a sense, we've developed herd immunity. It's not protecting people from getting an upper respiratory infection, but we're no longer seeing the severe consequences of a completely new respiratory virus coming into a totally naïve population."
COVID levels are trending downward after Massachusetts saw a bump around the holidays. Along with holiday parties, the new omicron subvariant XBB could very well have contributed to the spike, experts have said. The immune-evading variant has rapidly taken over as the dominant strain locally in recent weeks.
Things might look different if an entirely new variant emerged, Kuritzkes said. But each subvariant that has surfaced since the omicron surge continues to descend from the original strain, which prompted "astronomical" case numbers this time last year.
"I've been referring to it as an immunity wall as opposed to, you know, the image of the herd ... it's the wall that we have built through repeated infection or repeated vaccination," Tufts Medical Center's Dr. Shira Doron said. "We didn't want to have repeated infection, but we do and we can gain the benefits of that as a society and that is protecting us from big waves of severe disease."
The antibodies procured by natural infection and vaccination usually protect against future infection. If enough people in a population are immune, this leads to lower rates of prevalence of disease or viruses in a community. If a virus has less opportunity to spread and infect, it can be greatly controlled or even eradicated, the doctors said.
The classic concept of herd immunity would prevent COVID from circulating altogether, the experts noted, but it's not clear that we'll ever get to that point.
"It's not classic herd immunity," Boston Medical Center's Dr. David Hamer said. "It's immunity that's sufficiently high to prevent the bulk of the population from being hospitalized or having severe disease, but it's not going to stop people from being infected."