Early vaccine phases will likely ease the burden on hospitals, but may not slow the spread

A nurse at the Royal Cornwall Hospital prepares to administer COVID-19 vaccinations on Dec. 9 in Truro, a city in the United Kingdom.Hugh Hastings/Getty

Nearly 7 million people live in the 10,565 square miles that make up the Commonwealth. And it will take several months for even half of them to receive a vaccine that promises immunity against the coronavirus. But even if hundreds of thousands or a million Massachusetts residents are vaccinated by next spring, as Governor Charlie Baker suggested in a press conference Wednesday, the pandemic will still be a formidable threat.

So just how different might life be with, say, one out of every three Massachusetts residents vaccinated come early summer? Epidemiologists hope the initial phases of the vaccine rollout will lower death and hospitalization rates by providing immunity to the state’s most vulnerable populations. But that doesn’t mean the number of cases will fall at a meaningful rate, particularly if the general public becomes cavalier once vaccines become more available.

“There’s no question that as we start to pump this vaccine into the population we’re going to blunt this virus,” said Dr. Philip Landrigan, director of the Program for Global Public Health and the Common Good at Boston College. “It’s a more complicated question of what it’s going to do the spread of the disease in the population. If people in large numbers let down their guard and say, ‘Wonderful, the vaccine is here. We can go party,’ then we’re going to have a problem.”

The Baker administration aims to use the first shipments of the vaccine, arriving in the next few weeks, to inoculate health-care workers, emergency services personnel, and nursing home workers and residents. If all goes according to plan, the second phase will target high-risk individuals, teachers, grocery workers, and adults over 65. These two phases aim to vaccinate roughly 2.5 million residents — or 36 percent of the population — but right now the state has access to enough of the vaccine for only 1.1 million of those people to get the two doses necessary for immunization. Lastly, the rollout will commence for the remaining two-thirds of the population, which amounts to some 4.4 million people. The exact timing of each of these phases remains contingent on vaccine supply and efficacy.

But the virus will remain a threat until a large majority of the general public receives a vaccine. Herd immunity only occurs when so many people are immune to the virus that an infected person rarely contacts a non-immune person and, unable to find new hosts, the virus dies out. Measles, a notoriously contagious virus, slows down only after about 95 percent of people become immune. The exact herd immunity threshold for COVID-19 is unknown, but epidemiologists suspect it to be somewhere between 60 percent and 75 percent.

In the meantime, if unvaccinated folks ease up on public health measures such as mask-wearing and social distancing that are stemming the tide, the virus could continue to spread at an alarming rate even as vaccinations are taking place. Deaths and hospitalizations would not stop.

“That strategy would achieve herd immunity at a cruel price, because it would inevitably lead to unnecessary cases of disease and unnecessary deaths,” said Landrigan. “Remember, although rates of hospitalization and death in younger people are certainly lower than in older folks, rates of disease and death from COVID-19 infection in the young are certainly not zero.”

Indeed, adults ages 20 to 44 account for one-fifth of COVID-19 hospitalizations in the US, with 12 percent of those ending up in the intensive care units, according to data from Johns Hopkins. The virus landed a 25-year-old former Bates College athlete and a 49-year-old South Shore hiking aficionado in comas for weeks. It’s killed a 30-year-old New Jersey baseball coach and a 31-year-old karaoke and Patriots lover from Boston. Even mild cases among young adults have left them with residual symptoms and cardiovascular abnormalities that linger long after their initial diagnosis.

The novelty of these new vaccines only makes it more difficult to say how quickly vaccinations will usher in a sense of normalcy. It is possible, for example, that the vaccinated may still be able to transmit the virus, even if they don’t become ill themselves.

“We don’t know that [available vaccines] help prevent transmission, necessarily. If it really works to prevent all infection, including asymptomatic infection, that should help bring things back to normal quickly,” said Dr. David Hamer, a Boston University infectious disease expert and physician at Boston Medical Center. “But if, say, it helps protect against disease, but not infection — so people can become infected, shed virus, and then not become symptomatic because they’ve had the vaccine, but still can spread it on to other people — that would be a concern.”

Regardless, epidemiologists do not see the vaccinations mitigating the catastrophe that could occur if trends statewide and nationwide continue at their current rates. In just nine days, December already has seen daily case totals in Massachusetts that dwarf those of the spring months. There were 89 deaths in the state Wednesday, the most since June 1.

“Let’s not allow the news of vaccines blind us from putting the resources forward today for mitigation strategies in the near future, nor from continuing to press on the importance of continuing to improve currently available public health measures,” wrote Dr. Michael Mina, an epidemiologist at Harvard, in a tweet on Monday, injecting a dose of grim urgency into a public with raised hopes about the future.