Enough appointments for COVID-19 vaccinations were available Tuesday at the mass clinic in Bowie that Maryland Gov. Larry Hogan took to Twitter around 2 p.m. to urge people to go.

“Some additional appointments are available TODAY until 5:15 p.m. at the Six Flags America mass vaccination site. Go out and get vaccinated,” the governor tweeted.

Late Tuesday afternoon, hundreds of appointment slots remained available for a mass vaccination clinic Wednesday at the Timonium State Fairgrounds.

Gone are the days when appointments were nearly impossible to find. They’re now widely

available at Maryland’s mass vaccination clinics and a growing number of sites now accept walk-ups. But appointments aren’t going begging — yet. Even as the supply of vaccine expands, state health officials said the demand remains high at Maryland’s mass vaccination clinics.

Even with the expanded availability, the state says vaccine wastage “has been very low.”

“We are generally able to monitor draws against appointments and balance them well at the end of each day,” said David McCallister, a spokesman for the Maryland Department of Health.

As of Wednesday, providers had administered 4 million shots, according to health department data. Almost 1.7 million people in the state — nearly 28% of the population — are fully immunized, with many more awaiting the second dose of their regimen.

Hogan said Wednesday that state vaccinators have maintained a consistent momentum for weeks, but it’s not sustainable, as more and more of the state gets immunized.

“Right now, we’re continuing to move at the same pace, but we have to do more creative things to get to the same number we were before,” he said. “At some point soon we’re going to be shutting down mass [vaccination] sites. Everything’s going to be finding those last few people, knocking on doors, calling them on the phone.”

Some estimates call for 80% of the population or more to get vaccinated so as to reach herd immunity, the necessary threshold to end the coronavirus pandemic. Herd immunity is achieved when the virus runs out of viable hosts to latch onto and the number of daily infections, hospitalizations and deaths plunge.

The national vaccination campaign provides a pathway out of the public health crisis on the condition that most adults and children get one of the three authorized COVID-19 vaccines. Administration of the single-dose immunization made by Johnson & Johnson has been paused as federal regulators study six rare reports of severe blood clotting that may stem from the product.

Hogan said this month that the Johnson & Johnson pause would not substantially affect the vaccine operation in Maryland. The bulk of vaccinations are being supplied by Pfizer/BioNTech and Moderna, both of which call for a two-dose regimen.

In Maryland, more than half of the population 16 and older, including more than 80% of people 65 and older, have received at least one dose, state officials said. Clinical studies testing the vaccine in young children are currently underway.

Officials also said they are preparing for the “next phase” of the vaccination campaign, when demand dips at the mass sites, by increasing the number of walk-up appointments available and targeting their efforts toward groups considered difficult to reach or resistant to vaccines in general.

Supply already has outpaced demand in other parts of the country, especially in Southern states where vaccine resistance is high. National and state data show white men who lean politically conservative are generally more likely to decline a vaccine.

Dr. William Moss, executive director of the International Vaccine Access Center at the Johns Hopkins Bloomberg School of Public Health, said the country is approaching a critical “inflection point” as other states start to meet resistance in their remaining vaccination pools.

“The real focus in the United States is going to have to be addressing vaccine hesitancy and building trust, and trying to get those people vaccinated who at the present time are reluctant to do so,” Moss said. “And that’s going to be a long, more intensive process.”

There are 12 state-run mass vaccination clinics in the state, including two in Baltimore City, and those in Anne Arundel, Baltimore, Charles, Frederick, Harford, Howard, Montgomery, Prince George’s, Washington and Worcester counties. Pharmacies, retail partners, hospitals and federally qualified health centers also have been designated as providers.

All available channels should remain open in the coming months to provide easy access to shots for all consumers, said Sophia L. Thomas, president of the American Association of Nurse Practitioners.

She said misinformation and access continue to pose barriers. Meanwhile, she said, the likely temporary Johnson & Johnson vaccine pause has not helped improve confidence.

“We have to go to where the community is, and meet them in home environments, community centers, churches,” said Thomas, who’s also a certified family and pediatric nurse practitioner. “A lot of education is important as we do this, and it’s going to have to be from key, responsible messengers.”

The Greenbelt vaccination clinic run by the Federal Emergency Management Agency opened for walk-up service Tuesday, McCallister said. It joins M&T Bank Stadium in Baltimore as well as the sites in Hagerstown and Salisbury in not requiring registration.

On Thursday, the Baltimore Convention Center Field Hospital will begin opening 200 walk-up appointments a day specifically for city residents, and the site at Navy-Marine Corps Memorial Stadium in Annapolis will have walk-ups available as well.

“Multiple efforts are underway that are customized to hard-to-reach groups, including the addition of mobile vaccination clinics, targeting senior citizens through departments of aging, reaching out to individuals via phone and text messaging, and engaging primary care physicians to contact their patients,” McCallister said.

Such targeted efforts are critical pieces of the state’s vaccination campaign, which hinges on providing access to Marylanders of every age group and demographic. But so far the vaccines have gone disproportionately to white Marylanders, who likely have more access to health care, transportation and the internet than communities of color.

The state launched a Vaccine Equity Task Force last month aimed at closing such gaps in vaccinations. So far the unit, headed by Maryland National Guard Brigadier Gen. Janeen Birckhead, has administered more than 30,000 doses, at churches, synagogues and apartment buildings and with mobile clinics, according to the state.

Every community needs at least a portion of its population vaccinated, public health experts say, to avoid spreading the virus.