Does the smallpox vaccine work for monkeypox?
Discovered in 1958 in research colonies with monkeys, the first human case of monkeypox was recorded in 1970 in the Democratic Republic of the Congo. It is less transmissible and less dangerous than smallpox, killing only 3-6% of those infected. Unlike smallpox, monkeypox circulates among animals, making it especially difficult to eradicate the disease.
However, early indications suggested that people inoculated against smallpox might be more successful against monkeypox. In 1988, researchers in Zaire looked at the rates of monkeypox cases in people with and without smallpox vaccination scars. The study concluded that it was 85% effective in protecting against monkeypox.
MacIntyre says that historically, monkeypox outbreaks were rare and small, with case numbers in the single or double digits. But this has changed in recent years. A 2010 study in the Democratic Republic of the Congo found a 20-fold increase in human monkeypox cases, with a disproportionate number of infections in young people who had never received a smallpox vaccine.
“Then in 2017 we started noticing very large outbreaks in Nigeria and then in the Democratic Republic of the Congo,” says MacIntyre. His team’s research into Nigerian outbreaks between 2017 and 2020 found that the cases were linked to declining population immunity levels, especially in young people who had never been vaccinated against smallpox and in older people whose decades-long protection was beginning to disappear.
Even so, the theory that smallpox vaccines protect against monkeypox is not settled sciencewarns Wafaa El-Sadr, founder and director of ICAP, a global health institute at Columbia University. Although these studies suggest that older people who have been vaccinated against smallpox may have some protection, she notes that there is no data to definitively confirm this.
The knowledge gap is particularly wide when it comes to the new vaccine, Jynneos. The only studies done so far that have shown the efficacy of the monkeypox vaccine were done in animals, not humans, El-Sadr says. It is also unclear if Jynneos is safe for use in children., who are more susceptible to severe monkeypox. And, because the United States proposes to expand its vaccine supply by injecting smaller doses between the layers of the skin, rather than into the fat under the skin, data to support the move is sparse.
“I can list many, many questions that need answering,” El-Sadr emphasizes. “The good news is that we have a vaccine in hand that should in all likelihood be effective against monkeypox in humans.”
Who should get vaccinated against monkeypox?
The monkeypox outbreak does not mean that smallpox vaccinations will become routine again. After all, the decision to administer any vaccine must balance the risks against the benefits.
Jynneos may be safer than the older generation of vaccines as it contains no live virus, but it still carries some risk of side effects, such as flu-like symptoms or an allergic reaction.
“If there is no benefit (to receiving the vaccine), then any risk is not worth it,” Poland warns.
Experts also maintain that there is no urgency in the distribution of the vaccine as a preventive measureas was done with the COVID-19 vaccine. Smallpox vaccines are effective after exposure, so it makes more sense to prioritize people who are concerned they may have been infected.
Of course, this risk-benefit analysis could change depending on what happens next. “If it is limited and we are able to stop this outbreak in its tracks, then it is very difficult to say that we should recommend this vaccine for everyone,” El-Sadr said.
However, if the United States fails to contain monkeypox, more extensive vaccination may be needed, especially if it spreads to the animal reservoir and becomes endemic. Vaccination recommendations could also change if monkeypox begins to circulate widely among children, who are more at risk than adults.
El-Sadr is hopeful that this will not happen. “Fortunately, this virus is very different from smallpox, and the consequences of contracting monkeypox are very different from smallpox infections,” he says. “We have an outbreak, that’s true, and that’s a big concern. But we’re lucky to have a test at our fingertips that can diagnose monkeypox, a vaccine that we can use and hopefully scale up, and we also have a treatment.”