Does the ancient vaccination against smallpox protect against monkeys?

Health officials are trying to distribute insufficient doses of the vaccine to slow the spread of monkeypox, but are older adults vaccinated against the smallpox virus as children already protected?

The Centers for Disease Control and Prevention doesn’t bet on that. The U.S. health agency says those who have contracted monkeypox “should consider getting vaccinated if they have not received a smallpox vaccine in the past 3 years.” On Monday, the CDC approved a strategy to administer the smallpox vaccine subcutaneously — at one-fifth the usual dose.

Although a limited number of studies offer hopeful signs that childhood smallpox vaccinations offer lasting protection decades later, health experts say aging Baby Boomers who still carry signs of childhood vaccinations should not assume they are not immune.

“I wouldn’t say there’s no protection,” said Andrew Neumer, associate professor of population health and disease prevention at the University of California, Irvine. “But that’s not something I can count on.”

Smallpox, a highly contagious ancient scourge that kills three out of 10 people it infects and scars survivors, was eradicated worldwide in 1980 thanks to a global vaccination effort. The disease has been eradicated in North America and Europe since the early 1950s, and routine vaccinations in the United States ended in 1972.

The vaccine is based on a sibling of smallpox and monkeypox and offers protection against both. Although monkeypox is not as deadly as smallpox, it can still cause painful rashes and symptoms and permanent scarring.

The U.S. now leads all countries in global monkeypox outbreaks, with about 32,000 worldwide and about 12,000 in the U.S., including nearly 2,000 in California and 2,300 in New York.

In 1972, in Yugoslavia, 175 people were infected and 35 died, scientists who studied smallpox found that 105 people, or 60% of the patients, had been vaccinated against smallpox. However, they noted a large difference in mortality rates: 8% among those previously vaccinated and 35% among those not vaccinated.

According to Neumer, the study shows that the vaccine’s ability to prevent disease has diminished over the decades. He noted that before smallpox was eradicated, the CDC recommended revaccination every five years.

The estimate that smallpox vaccines are 85% effective against monkeypox infection is based on a 1988 study in Zaire, where smallpox vaccination ended in 1980. Although the study was good, it was not a clinical randomized controlled trial, Neumer said.

Seth Bloomberg, an assistant professor of medicine at UC San Francisco, said research following the recent monkeypox outbreak in the U.S. has yielded positive results. The 2003 outbreak was spread through domestic animals kept near infected African animals. The study found that people who had previously been vaccinated against smallpox were among those infected, but the disease may be milder.

“Although immunity can wane over time, there appears to be immunity to particularly severe disease,” Bloomberg said.

Another 2010 study looked at the recurrence of monkeypox in the Democratic Republic of the Congo and found that since routine vaccinations ended, there has been a “dramatic increase in human monkeypox in rural Africa three decades after massive smallpox campaigns stopped.” It also found that “vaccinated individuals were 5.2 times less likely to develop monkeypox than unvaccinated individuals.”

“There were no clear signs that the vaccine was making it less effective,” Bloomberg said. “It’s very reassuring.”

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