Men eligible for the monkeypox vaccine who did not receive one were roughly 14 times more likely to catch the virus, early real-world effectiveness data from the CDC showed.

During the week of August 28, for example, incidence of monkeypox was 230 per 100,000 for those who did not get vaccinated, as compared to 15 per 100,000 in those who received at least a single dose of the Jynneos vaccine, according to findings highlighted by the White House Monkeypox Response Team on Wednesday.

Vaccine protection was seen as early as 2 weeks after the first dose, said CDC Director Rochelle Walensky, MD, MPH, who noted that real-world effectiveness data for the two-dose vaccine had been limited.

The analysis was restricted to males ages 18 to 49, and matched preliminary vaccine data with monkeypox case data from July 31 to September 3 across 32 states (representing 54% of the U.S. population). Limitations included that the analysis could not account for potential differences between the vaccinated and unvaccinated groups when it came to testing or behavior.

“These early findings and similar results from studies in other countries suggest that even one dose of the monkeypox vaccine offers at least some initial protection against infection,” Walensky said.

However, laboratory studies of the Jynneos vaccine have shown that immune protection is highest 2 weeks after the second dose of vaccine, said Walensky, and stressed the importance of eligible individuals receiving the complete two-dose regimen 28 days apart, “to ensure durable, lasting immune protection against monkeypox.”