The spread of monkeypox around the world could be stopped by vaccinating less than half of high-risk men who have sex with men, according to a modelling study.
But there is a lot of uncertainty around the numbers used in the model, which also makes optimistic assumptions about the effectiveness of monkeypox vaccines, says at Yale School of Public Health, a co-author of the study.
Amid the ongoing outbreak, . For the first time ever, deaths have been reported outside of the Central and West African countries where the disease is endemic. As of 4 August, Spain had reported two deaths and Brazil, Peru and India had one death each. One of those who died was immunocompromised because of cancer.
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The virus is mainly spreading among men who have sex with men, through close physical contact or by touching clothes, bedding or towels used by people who are infected. In the UK, for instance, .
Initial attempts to contain the outbreak were based solely on tracing contacts of people with monkeypox to identify possible cases early and prevent them passing on the virus. As it became clear these efforts were insufficient, countries including the UK and US also began offering vaccinations to those at high risk of infection.
To assess what level of vaccination is needed to stop the outbreak, Gonsalves, his co-author , also at Yale School of Public Health, and their colleagues adapted a model developed for covid-19.
The model assumed that detecting a case at any point of the infection halves the risk of that person infecting others, while detecting cases early in the infection鈥檚 onset via contact tracing specifically reduces further infections by 90 per cent.
The researchers also assumed that monkeypox vaccines are 100 per cent effective at preventing infection, when their actual effectiveness isn鈥檛 known. 鈥淭here is so much uncertainty here,鈥� says Gonsalves.
Based on this, the model suggests that between 5 and 47 per cent of high-risk men who have sex with men need to be vaccinated to stop the outbreak.
The precise numbers depend on how many other people each person with monkeypox infects on average 鈥� known as the basic reproduction (R0) number 鈥� as well as what proportion of cases are detected and how many of those cases are contact traced.
If monkeypox鈥檚 R0 number is 1.6, 20 per cent of cases are detected and 25 per cent of those cases are contact traced, then 29 per cent of high-risk individuals need to be vaccinated.
Based on national surveys of the US, the team estimates there are 500,000 high-risk men who have sex with men in the country. This would mean vaccinating 145,000 people. The main vaccine being used 鈥� called Imvanex, Imvamune or Jynneos 鈥� requires two doses, so 290,000 doses would be needed in this instance.
Unlike most countries, the US has enough doses to achieve this, but reaching these vaccination levels might still be difficult. In big cities in the US, there have been long queues for vaccination, says Gonsalves, but it isn鈥檛 clear if all those people are the high-risk men who really need the vaccine. In rural areas, men may be less willing to come forward, he says.
鈥淚 think that the percentage vaccination levels we report are really a floor, that in reality we are going to have to vaccinate a lot more people to ensure we are covering these high-risk men and to make up for the vaccine not being perfectly efficacious,鈥� says Chitwood.
While case numbers in the US are growing exponentially, in some of the first countries to report cases, such as the UK, numbers , suggesting containment efforts there are starting to work.
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