Smallpox vaccinations may not protect against monkeypox for life, research suggests, with experts saying HIV may play a role in eroding protection from the jab over time.
Monkeypox outbreaks are ongoing around the world, with the World Health Organization declaring the disease a public health emergency of international concern. At present, the majority of cases in current outbreaks are among men who have sex with men.
Vaccination with a jab initially developed to protect against smallpox, a related but more serious disease, is among the measures being taken to control infections.
However, while experts stress that it is important for those at risk of monkeypox to take up the offer of a vaccination, as it reduces the chance of symptomatic infection and severe illness, protection offered by a smallpox jab may decline over time. A study into monkeypox cases in Spain revealed that 32 of the 181 patients had previously received a childhood vaccination against smallpox.
Dr Oriol Mitja, co-author of the research, said that since most participants who had been vaccinated against smallpox received the jab more than 45 years ago, it is reasonable to predict that their protection would have waned. “All I can say is that childhood vaccinations may not protect 100% for life,” he said.
Jimmy Whitworth, professor of international public health at the London School of Hygiene & Tropical Medicine, and who was not involved with the work, agreed.
He suggested that there could be a number of reasons at play, including that while the viruses are similar they are not identical, “so the cross-protection provided may not absolute”, he said.
Furthermore, Whitworth noted that the study relied largely on self-reported smallpox vaccination, meaning there may have been inaccuracies.
However, Mitja said most clinicians had also checked scars, vaccination cards, or the patient had asked his mother.
Another possibility, said Whitworth, is that HIV may play a role. According to the study, 40% of the monkeypox cases were in people who were HIV positive. Mitja said the figure was 60% among those who had childhood smallpox vaccination but still got monkeypox. “[People with HIV] may have had some immunodeficiency, eroding away the protection from the vaccine,” said Whitworth.
Laura Waters, the chair of the British HIV Association, agreed. “Though it is likely smallpox vaccine effectiveness wanes in everyone, it’s feasible that this would occur to a greater degree in people with HIV, even those with well-controlled HIV on treatment,” she said.
Research from scientists in the US, published in 2020, found immune responses to childhood smallpox vaccination declined faster among people who subsequently became infected with HIV.
Prof Mark Slifka, of Oregon Health & Science University, said: “This is a potential concern that may explain why there could be more monkeypox breakthrough cases in these current outbreaks.”
But he urged caution in interpreting the data from Spain, noting that the childhood smallpox vaccine might still have provided partial immunity towards monkeypox.
“We also don’t know if the cases among previously vaccinated individuals was less severe compared to those who were not previously vaccinated,” Slifka said, noting another study by his team looking at an earlier monkeypox outbreak in the US suggested childhood smallpox vaccination reduced the chance of catching monkeypox.
A spokesperson for the Terrence Higgins Trust said more research is needed on the effectiveness of the vaccine in people with HIV, adding that the charity is calling for the UK Health Security Agency to investigate whether people with HIV need a second dose of the vaccine. Limited supplies of the vaccine mean that at present only one dose is being offered to those at risk of monkeypox.
The smallpox vaccine being used in many countries, including in the UK, is not the same as the one given decades ago. Known as Imvanex in the UK and Jynneos in the US, the jab does not contain live virus, unlike earlier vaccines, making it safe for people with HIV.
Dr Carlos Maluquer de Motes, a virologist at the University of Surrey, said the current monkeypox outbreak would offer important data on how long immunity offered by the smallpox vaccine lasts.
“No studies have been able to measure ‘real’ protection from [smallpox] simply because there was no disease once smallpox was eradicated,” he said. With monkeypox closely related to smallpox, the current outbreak could offer fresh insights.
Dr Maluquer de Motes added: “Whilst we think most individuals are protected, natural individual-to-individual variation in vaccine response efficacy is to be expected and some individuals may still be susceptible to monkeypox disease. This picture will emerge as numbers increase and larger studies are conducted.”