Since the early days of the COVID-19 pandemic, health officials and doctors alike have held up “herd immunity” as a goal for overcoming the crisis.

This happens when a large portion of a community is immune to a disease — typically through vaccination — making further spread unlikely. So with nearly half of New Hampshire residents fully vaccinated against COVID-19 and cases consistently dropping, when will we hit that sweet spot?

According to health experts, it’s hard to say.

“There’s no objective definition of herd immunity, and it’s more of a continuous function, as we say, rather than a threshold. It’s not like a light switch; it’s more like a dimmer,” said Dr. William Schaffner, a professor of medicine in the Division of Infectious Diseases at Vanderbilt University in Tennessee.

The percentage of the population that needs to be immune to achieve herd immunity varies by disease and how contagious it is.

For example, measles spreads so easily that an estimated 95 percent of the population must be vaccinated to attain herd immunity, according to the World Health Organization. The remaining 5 percent are then also protected because the virus will no longer be able to spread.

Most estimates for COVID-19 set that number at between 70 and 80 percent immunized. But with vaccine hesitancy, the emergence of new variants and delayed vaccine eligibility for children, many health experts say it’s unclear whether herd immunity will ever be achieved for COVID-19.

“The question is, ‘What’s the goal?’ Well, if we want to get down to really low levels of virus transmission and really low levels of cases so that we can say, ‘Yes, we’ve got this under control,’ then we do need to get up to around 80 percent of people vaccinated,” Schaffner said.

Dr. Michael Lindberg, who served as chief medical officer at Monadnock Community Hospital in Peterborough before retiring in April, said it’s also crucial to know how many people in New Hampshire and beyond have already contracted the virus.

“The number we don’t know is what part of the population had the virus and was infected and didn’t know,” he said. If, for example, “20 percent of the population got infected but didn’t know it, that could bump us to that 70 percent,” he continued. “But I don’t think we are there yet.”

Herd immunity is not only beneficial for keeping transmission low, Lindberg added, but also to prevent more COVID-19 mutations from developing.

“Over time, if it spreads, we run the risk of developing more variants … Every time the virus jumps to another person, that chance for a mutation occurs, and we don’t want to see that,” he said.

Some variants of COVID-19 are thought to be more contagious, which could increase the risk of infections, hospitalizations and deaths. Additionally, new variants could be resistant to vaccination.

Dr. Daniel Perli, the Peterborough hospital’s current chief medical officer, said the recent relaxation of safety protocols — like allowing vaccinated people to take off their face masks inside — is reasonable, especially with the current case numbers.

But, he said, whether or not we’ll achieve herd immunity for COVID-19 is “complex.”

“It’s unclear, but to be honest it seems less likely that we would,” he said. “We still don’t know enough about herd immunity with COVID and if it’s achievable or not, and ... we may never achieve enough immunity levels in the community to [get] rid of the virus completely.”

Regardless, he and Lindberg stressed the importance of people getting vaccinated, which will still lower the virus’ transmission.

“We don’t want the public to feel like they shouldn’t continue getting vaccinated because we aren’t going to hit this magic herd-immunity number, because the idea is to prevent transmissibility, protect the vulnerable and to reduce hospitalizations and mortality,” he said. “That’s the idea we should focus on.”

Olivia Belanger can be reached at 352-1234, extension 1439, or Follow her on Twitter @OBelangerKS.