Smallpox vaccines may not protect against smallpox for life, research shows, and experts say HIV may play a role in eroding protection against smallpox over time.
Monkeypox outbreaks are ongoing worldwide, with the World Health Organization declaring the disease a public health emergency of international concern. Currently, the majority of cases in the current outbreaks are among men who have sex with men.
Vaccination with a shot originally 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 people at risk of monkeypox to take up the offer of a vaccine, as it reduces the chance of symptomatic infection and serious illness, the protection offered by a smallpox shot may decrease over time. A study of cases of monkeypox in Spain revealed that 32 of 181 patients had previously received childhood vaccination against smallpox.
Dr. Oriol Mitja, co-author of the research, said that because most of the participants who had been vaccinated against smallpox received the vaccine more than 45 years ago, it is reasonable to predict that their protection would have waned. “All I can say is that childhood vaccines may not protect 100% for life,” he said.
Jimmy Whitworth, professor of international public health at the London School of Hygiene & Tropical Medicine, who was not involved in the work, agreed.
He suggested that there could be several reasons at play, including that while the viruses are similar, they are not identical, “so the cross-protection provided may not be absolute,” he said.
In addition, Whitworth noted that the study was largely based on self-reported smallpox vaccination, which means there may be inaccuracies.
However, Mitja said most doctors had also checked the scars, vaccination cards or the patient had asked his mother.
Another possibility, Whitworth said, is that HIV may play a role. According to the study, 40% of monkeypox cases were in HIV-positive people. Mitja said the figure was 60% among those who were vaccinated against childhood pox but still had monkey pox. “[People with HIV] may have had some immunodeficiency, eroding the vaccine’s protection,” Whitworth said.
Laura Waters, president of the British HIV Association, agreed. “Although the effectiveness of the smallpox vaccine is likely to be reduced in everyone, it is likely that this will happen to a greater extent in people with HIV, even in those whose HIV is well controlled on treatment,” he said.
Research by US scientists, published in 2020, found that immune responses to childhood smallpox vaccination declined more rapidly among people who later became infected with HIV.
Professor Mark Slifka of Oregon Health and Science University said: “This is a potential concern that may explain why there might be more cases of smallpox breakthrough in these current outbreaks.”
But he urged caution in interpreting the data from Spain, noting that the childhood smallpox vaccine may still have provided partial immunity against monkeypox.
“We also don’t know if the cases among people who were previously vaccinated were less severe compared to those who were not previously vaccinated,” Slifka said, noting that another study by his team that looked at a previous smallpox outbreak in the U.S. United suggested that childhood smallpox vaccination be scaled back. the possibility of catching monkey pox.
A spokesman for the Terrence Higgins Trust said more research was needed into the effectiveness of the vaccine in people with HIV, adding that the charity was asking the UK’s Health Safety Agency to investigate whether people with HIV needed a second dose of the vaccine. Limited supplies of the vaccine mean that only one dose is currently offered to people at risk of smallpox.
The smallpox vaccine used in many countries, including the UK, is not the same as that given decades ago. Known as Imvanex in the UK and Jynneos in the US, the jab contains no live virus, unlike previous 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 will provide important data on how long the immunity provided by the smallpox vaccine lasts.
“No study has been able to measure ‘real’ protection against [smallpox] simply because there was no disease once smallpox was eradicated,” he said. With monkeypox closely related to smallpox, the current outbreak could offer new insights.
Dr Maluquer de Motes added: “Although we believe that most people are protected, natural variation from individual to individual in the effectiveness of the vaccine response is to be expected and some people may still be susceptible to monkeypox disease. That picture will emerge as the numbers rise and larger studies are done.”