U.S. has stockpiled millions of doses of two smallpox vaccines, also effective against monkeypox. But the outbreaks so far are clustered in other countries.

As more than a dozen countries grapple with outbreaks of monkeypox, health officials worldwide are rushing to assess reserves of vaccines and treatments that may be needed to contain the spread.

The U.S. emergency stockpile holds two vaccines approved by the Food and Drug Administration that could be used to contain monkeypox, officials at the Centers for Disease Control and Prevention told reporters on Monday.

The stockpile contains more than 100 million doses of the original smallpox vaccine. But that vaccine is associated with side effects and shouldn’t be given to certain patients, including those who are immuno-compromised.

A newer vaccine, called Jynneos, was approved in 2019 for prevention of both smallpox and monkeypox. More than 1,000 doses are held in the stockpile, Dr. Jennifer McQuiston, a deputy director at the C.D.C., said.

“We expect that level to ramp up very quickly in the coming weeks, as the company provides more doses to us,” she said. Doses have already been requested from the stockpile for inoculation of some high-risk contacts, she added.

Still, the dimensions of the problem in the United States are not yet clear. As of Monday, officials had confirmed just one case, in Massachusetts, and were evaluating four other patients.

The situation overseas is more concerning. As of Monday, there were more than 100 confirmed cases in 14 countries outside Africa, and dozens more under investigation. That day, the European Center for Disease Prevention and Control urged health officials in Europe to assess the availability of smallpox vaccines, antivirals and personal protective equipment.

The World Health Organization is holding stockpiles of about 31 million smallpox vaccine doses, but they may have lost some potency in the decades since they were made.

The largest monkeypox clusters have been reported in Europe, particularly in Spain, prompting some experts to hypothesize that the outbreaks originated in that country. Spanish officials are investigating two potential sources: a Gay Pride event held in the Canary Islands beginning on May 5 that drew about 80,000 revelers, as well as a sauna in Madrid.

Although many of the initial infections in Europe were reported among men who have sex with men, other segments of the population are clearly vulnerable. Officials “must keep a broad definition, so as not to leave out other groups which may emerge as being important,” said Dr. Boghuma Titanji, an infectious diseases physician at Emory University in Atlanta.

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