As cases of the disease caused by the mpox virus increase around the country and in Massachusetts, health experts are encouraging at-risk people to get vaccinated.

The first documented case of the virus in the United States was found in Massachusetts in May of 2022, and by the end of that year nearly 500 cases had been documented in the state.

Last year, things seemed to be more under control as mpox was found in just six people in Massachusetts. Already this year, there have been 16 mpox cases in the state.

“We’re concerned that the 16 that we have here can explode into hundreds of cases,” said Dr. Cassandra Pierre, an infectious disease physician who is the associate hospital epidemiologist and medical director of public health programs at Boston Medical Center.

“Mpox is not a sexually transmitted disease, but it is associated with sexual activity,” Pierre said. “The majority of individuals who we’ve seen infected with mpox are gay, bisexual and other men who have sex with men. Over 90% of the cases have been reported in that population.”

The virus can cause painful skin lesions and other symptoms, which can last for several weeks. (While the virus itself is still commonly called monkeypox, the WHO recently changed the name of the disease it causes to mpox.)

“We’ve had conversations with the DPH already earlier this year about this issue as we started to see more people who were infected and unvaccinated for mpox,” Pierre said, referring to the Massachusetts Department of Public Health.

Mpox vaccines are now widely available, said Dr. Kevin Ard, director of the sexual health clinic at Massachusetts General Hospital.

“There is a DPH website that can show people where they can get vaccines, and we provide the vaccine at no cost right now,” Ard said. “So, there’s really no supply limitation like there was very early, when the outbreak first began.”

Nationally, Ard said, low vaccination rates are contributing to an increased risk of another outbreak.

“The vaccine rates in Massachusetts are higher than the national average, so I think our risk for an outbreak is probably lower,” he said. “But still in Massachusetts, there are many people who are eligible for the vaccine who haven’t had it.”

The number of cases in the 2022 outbreak spiked in the summer, Pierre said, raising concern that a similar surge could be seen again.

“We are concerned that, going into the summer, there remain a large population of at-risk individuals who are unvaccinated and therefore at risk,” Pierre said.

It takes two doses of the vaccine for people to be considered fully vaccinated, Ard said.

“People can protect themselves by getting vaccinated if they’ve not had any vaccines,” Ard said. “And if they’ve only had one vaccine, they should go ahead and get the second vaccine because we think that that provides the highest protection.”

Mass General Hospital plans to offer free vaccinations at Boston Pride events over the next month, Ard said.

Both Pierre and Ard also expressed concerns that a more dangerous variant could be on its way.

In a report, the CDC issued a warning earlier this month about “clade I.”

The type of virus that’s been circulating in the United States so far is known as “clade II,” but another variant, known as “clade I” is currently having an outbreak in the Democratic Republic of Congo. That variant causes more severe illness and is associated with higher rates of mortality. No cases have been detected in the United States.

“We are concerned about the threat that clade I poses to the population at risk,” Pierre said. “Especially higher risk people who are unvaccinated.”