A U.S. Navy hospitalman prepares a dose of the COVID-19 vaccine to be administered to a patient at Walter Reed National Military Medical Center in Bethesda, Maryland, in this December 2020 file photo. Photo by Lisa Ferdinando | Department of Defense

As the omicron variant continues to drive rapid spread of the coronovirus, some public health experts warn that the Baker administration has not done enough to get booster shots into arms.

New infections increased at a rapid pace over the past few months, and while the state announced it had delivered booster vaccinations to over 2 million Massachusetts residents since the rollout began in late September, about six in 10 Bay Staters who completed an initial vaccine sequence still have not gotten the extra shot.

That rate is worrisome for administration critics, who point to growing research that the standard vaccine dosage alone is insufficient to defend against the omicron variant driving most transmission across the country.

“The efforts of the state have been grossly insufficient since the early summer. They haven’t ramped up for the winter surge that was foreseeable in late September, and they’re grossly insufficient now,” Julia Koehler, a pediatric infectious disease clinical specialist and assistant professor of pediatrics at Harvard Medical School, told the News Service. “Forty percent [boosted] is not going to stop the omicron variant from ripping through the other 60 percent.”

2M Boosters So Far

Booster shots became available at the end of September, initially for residents 65 and older and some other vulnerable groups if they had received the Pfizer vaccine. Eligibility expanded to the general adult population on Nov. 18.

The U.S. Centers for Disease Control defines “fully vaccinated” as anyone who received both doses of the Moderna or Pfizer vaccine at or the single-shot Johnson and Johnson vaccine. In both cases, people must wait two weeks after their final shot to gain full immunization status.

Through Tuesday, the Department of Public Health counted 5,071,253 people in Massachusetts who are fully vaccinated. Among that group, 2,015,376 people, or nearly 40 percent of the population that completed a standard COVID-19 vaccination sequence, have received an additional booster dose.

That rate has more than doubled since the public became broadly eligible for boosters: on Nov. 18, less than 18 percent of fully vaccinated residents had also received a booster shot, DPH said.

Infections Grow Faster than Boosters

However, omicron’s arrival reshaped the public health outlook.

The state stands in the midst of an aggressive surge of coronavirus cases, even as widespread vaccinations have limited the number of deaths and the severity of illness. As of Tuesday, hospitalizations have more than tripled to 1,707, the seven-day average jumped more than five times to 5,446, and the positive test rate has exceeded levels observed during last winter’s peak.

The number of new booster shots administered continues to surpass newly confirmed infections, but the rates of change are not equal.

DPH reported 162,362 more booster shots in the week ending Dec. 28, down from the 312,628 doses in the week ending Dec. 14 in a slowdown likely due to the holiday season and an increase over the 138,338 shots administered the week ending Nov. 30.

Meanwhile, officials tracked 16,448 new infections the week ending Nov. 30 and 49,110 new infections the week ending Dec. 28, a nearly threefold increase.

Boosters Vital to Omicron Battle

Experts continue to study omicron to gain a better understanding of the latest variant, but early findings indicate the boosters will play a vital role in immunization. Koehler said the standard two-dose Moderna or Pfizer vaccine and the single-shot Johnson and Johnson vaccine provide “very poor protection” against omicron.

“It’s more than zero, but it is not really what we need,” she said. “Omicron has evolved to escape many of the immune mechanisms against the parent coronavirus, SARS-CoV-2, so that is why our standard dosing regime that up to now we had called fully vaccinated is no longer sufficiently protected.”

Gov. Charlie Baker and his deputies have repeatedly said that vaccinations and booster shots are the best options available for residents to protect themselves from the highly infectious virus and its newer variants.

“There are going to be a lot more cases because omicron is very contagious, but people need to understand that the vaccines and the number of people in Massachusetts that have gone out and gotten vaccinated and gotten boosted – their risk is extremely low,” Baker said in a GBH News interview last week. “It’s important for everybody to understand that vaccines and boosters are in fact doing exactly what they were supposed to do, and they are our best defense.”

In a report published last week, DPH estimated that booster shots have had a sizable impact in preventing infections. Between Oct. 31 and Dec. 4, the state recorded 21 cases per 100,000 fully vaccinated and boosted residents, compared to 145 cases per 100,000 residents who were fully vaccinated but had not received a booster shot.

Communities of Color ‘Underresourced’ for Fight

But some experts and community leaders argue that the administration has not done enough to drive home the importance of booster shots in its public messaging nor made the shots sufficiently accessible, particularly in communities with larger shares of people of color or families living in poverty.

“Overall, as in the rest of the country, the booster shot rollout has been disappointing,” said Massachusetts Public Health Association Executive Director Carlene Pavlos. “There’s been a lack of urgency and it’s been confusing, I think, to the general public about how important that need for booster shots is. It’s been far too difficult to get an appointment when people have wanted them.”

The Baker administration next week will reopen a Fenway Park vaccination site and launch state-sponsored sites in Lynn, Taunton and Roxbury, collectively adding capacity to offer 2,500 vaccines or boosters per day. They will add to a vaccine and booster clinic already open at the Whittier Street Health Center in Roxbury that has 500 doses available per day.

Pavlos said those sites have some advantage, but she contended that embracing mass vaccination sites as the administration did earlier in the pandemic will create obstacles for some populations.

“It doesn’t seem as if we learned from the mistakes of that first rollout in creating some strategies about how to reach communities that have been underresourced, underserved and overimpacted by COVID,” Pavlos said. “Overreliance on that strategy creates a real risk of inequity.”

Koehler voiced a similar concern, saying the state’s booster shot management has been “insufficient for the communities that, even for the first two doses, have not had the needed access and the needed messaging.”

“Towns like Newton, Needham, Brookline and Weston, where people have time and the habit of seeking out health information, they will seek out that information on their own and seek out the booster. Residents of these towns have time and ability to do it,” Koehler said. “Communities that have been left behind in vaccine access and vaccine information are dramatically left behind and uninformed about the need for a third dose.”

An administration official who agreed to communicate only on background said state-contracted mobile providers have held 1,638 clinics and administered 106,000 doses in the 20 “equity” cities and towns that have received additional state support during the pandemic. Providers administered more than 6,800 booster shots in those communities the week ending Dec. 19, the official said.

The official also said there are 200 booster locations in Boston, including 74 in Roxbury, Mattapan, Dorchester and Hyde Park, and cited a Kaiser Family Foundation report ranking the 73 percent of Massachusetts Black residents who have received at least one dose of a COVID-19 vaccine as the highest in the country.