Previous smallpox vaccine provides immunity to mpox, finds study

Vaccines against smallpox given until the mid-1970s offer continuing immunity against monkeypox, a study conducted in Sweden has found.

During last year's mpox (formerly monkeypox) outbreak, the virus spread for the first time outside Africa, causing over 85,000 cases of the disease to date.

Men who have sex with men account for the most infections, with a marked skew towards the young, the researchers said.

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The virus that causes mpox is known as an orthopoxvirus and is very similar to the virus that caused smallpox until the mid-1970s when it was finally eradicated.

Since there was data indicating that the old smallpox vaccine could protect against mpox, the researchers at Karolinska Institutet in Sweden wondered if the individuals who were vaccinated decades ago against the former would have some protection against the latter owing to a remaining memory response.

"Our study shows that this is the case, which implies that the memory cells are very long-lived and that they can recognise closely related viruses such as the mpox virus and provide overlapping, or cross-reactive immunity," said the study's corresponding author Marcus Buggert, researcher at Karolinska Institutet.

By analysing the T-cell immune response in 105 healthy blood donors, the study, published in the journal Cell Host & Microbe, showed that individuals born before 1976 had a significantly stronger immune response against both viral types.

The researchers also analysed the immune response in 22 men with a recent mpox infection and showed that they also exhibited a strong immune response to the virus, which may provide future immunity.

The current study was too small to judge how much protection previous smallpox vaccination provides, but Buggert refers to a recently published British observational study examining the effect of a smallpox vaccine given to risk-group males in 2022.

"This study shows that smallpox vaccine can provide about 80 per cent protection against mpox,” he said.

Mpox is a viral infection spread mainly through close physical contact with an infected person. Sexual contact poses a particularly high risk.

Common symptoms are blistering, sores and rashes, fever, and swollen glands. It can also cause pain and discomfort but typically clears up on its own after two to four weeks.

In Sweden, the smallpox vaccination programme started in the early 19th century and was discontinued in 1976 when the disease was eradicated.

The vaccine was mandatory for the entire population. The vaccine currently given for mpox is essentially a smallpox vaccine.