“I’m over 55 and was vaccinated against smallpox. I’m protected against monkeypox — right?” Well… not so fast.

In the emergent worldwide epidemic of monkeypox, the United States has the most cases of any country (now more than 10,000). Florida has the third-highest case count of any state (fifth-highest per capita). Some states, including California and Illinois, have declared health emergencies over the outbreak, while Florida is covered by the national emergency that the department of Health and Human Services declared following the states’ lead.

With people lining up around the block for monkeypox vaccine, there has been talk about the protection enjoyed by Americans born before 1972, when routine smallpox vaccination stopped in the United States. Can Americans — baby boomers, mostly — relax if they were vaccinated against smallpox as kids? Maybe, but we are health scientists who have been observing the epidemiology of monkeypox for years, and we are concerned about complacency.

The first thing to understand is that the smallpox vaccine is made of the vaccinia virus, which offers cross-immunity to many viruses related to smallpox, including monkeypox. Because it’s not made of smallpox virus, there is no risk of smallpox infections from the vaccine (a consequence we recently saw in the New York polio case).

Vaccinia-based vaccines will protect against monkeypox, but exactly how much is unknown. There has never been a randomized clinical trial measuring the effectiveness of the vaccine against monkeypox. The figure that has been making the rounds (85% effectiveness) is estimated from a single historical study from the 1980s.

Before smallpox was eradicated, the World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC) advised revaccination at five-year intervals, and this has been shortened to three years for modern lab workers who handle pox viruses. We simply cannot assume that any 50-year-old vaccinations are still protective.

Smallpox was eradicated through a massive public health effort that was built upon outbreak detection (a sort of meta-level contact tracing), in addition to an impactful vaccine.

As we age, our immune systems work less well, and most Americans vaccinated against smallpox as children are now in retirement or rapidly approaching it. This is another factor that should give us pause about assuming older Americans should rest easy with a decades-old smallpox vaccine scar.

While cases so far have been concentrated in people under age 50, this is an artifact of how the virus has been moving through extended social networks of men who have sex with men — stemming from superspreader events in Europe that helped jump-start the current outbreak — and cannot be assumed to be due to protection from any childhood smallpox vaccinations.

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Monkeypox spreads by skin-to-skin contact — not just sex — so it will not be confined to population groups usually associated with sexually transmitted diseases. In time, we expect to see more monkeypox cases in the broader population, including children and young adults. Monkeypox is a disease to avoid; though mortality is very low, the symptoms include fever, and a rash that is temporarily extremely painful with the potential for permanent scarring.

The CDC may eventually need to procure sufficient Jynneos (the newest and most side effect-free vaccine) for everyone, senior citizens included. Monkeypox protection, including good hand hygiene, is critical to keep this emerging virus in check.

Dr. Andrew Lover (Twitter: @AndrewALover) is an assistant professor of epidemiology, in the School of Public Health and Health Sciences, at University of Massachusetts-Amherst. His research over the last decade has focused on a broad range of public health programming for infectious disease control and elimination.

Dr. Andrew Noymer (Twitter: @AndrewNoymer) is an associate professor of population health and disease prevention at the University of California, Irvine. His research focuses on mortality from infectious diseases and quantitative-historical studies of pandemics.