A reliance on vaccines in short supply and questions over their best use are hampering efforts to curb the global spread of monkeypox which has hit dozens of countries for the first time, health officials say.
There are just 1.5 million vials of Bavarian Nordic's monkeypox vaccine either administered or currently available in the ten worst-affected countries, which account for nearly 90 per cent of all cases. The vast majority of doses are in the United States. The figures are based on a Reuters tally of government statements, and highlight the extent of the global supply gap for the first time.
The World Health Organization (WHO) estimates 10 million doses will be needed to protect the people at highest risk, with a current focus on certain men who have sex with men and contacts of people infected.
Other experts say an equitable global response would also prioritise an undetermined number of high-risk people in the 11 countries in Africa where monkeypox has been a public health problem for years.
Several countries, including the United States, Britain and Spain, are stretching out the available doses, with unknown outcomes.
Indeed, the Bavarian Nordic shot has not undergone clinical trials to evaluate the vaccine's ability to prevent monkeypox in humans, though initial studies suggest it will provide some protection.
"The whole vaccination strategy for monkeypox is associated with a lot of uncertainties," said Dr Dimie Ogoina, a professor of medicine at Niger Delta University in Nigeria and member of the WHO's monkeypox emergency committee.
Ogoina's research in 2017 highlighted possible sexual transmission of monkeypox in Nigeria – where no vaccines are, or have been, available.
"Unfortunately, I am not sure health authorities are communicating these uncertainties to the public," he added.
More than 40,000 people have already been infected by monkeypox this year, a virus which spreads through very close contact.
Around 10 per cent have been hospitalised with severe pain and 12 people have died. The WHO has declared the outbreak a global health emergency, its highest level of alert.
Disease experts are wary of the heavy emphasis wealthy countries have placed on vaccination as the first line of defence given these questions, as well as signs that behavioural change, testing and contact tracing are also important.
New monkeypox infections appear to be plateauing in Britain, Spain and Germany, a trend attributed by experts to behavioural change in at-risk groups.
"The first control strategy is behaviour change: asking people at high risk to reduce activities that are high risk, such as anonymous hook-ups and/or sex parties, and to adopt behaviours that may reduce risk, such as use of condoms, screening of your body for rash and for your sex partners' bodies for rash before sex," said Jay Varma, director of the Cornell Center for Pandemic Prevention and Response in the US
"The problem with this approach is that it's not durable."
Countries are setting up trials to assess the impact of the vaccine, including the US, Britain, Germany and Spain, although it is unclear when results may come out.
Bavarian Nordic's vaccine - called Jynneos, Imvanex and Imvamune, depending on the country - is the sole shot authorised by regulators in the U.S, Europe and elsewhere to prevent monkeypox.
Last week, the Danish company said it had signed on a US-based manufacturer to help accelerate delivery of doses.
The company was initially producing one batch - between 200,000 and 300,000 doses - per week, but since the start of this outbreak in May output has doubled, a spokesperson told Reuters.
Bavarian Nordic expects in-house capacity to treble by the end of 2022 from what it was in May - and is also in conversations with multiple other companies to expand manufacturing capacity, the spokesperson added.
Meanwhile, countries are trying to stretch out supplies.
Britain, Canada, and Germany are administering one dose per person instead of two, which allows them to inoculate more people even if they each may receive less or less durable protection.
In the United States and Europe, there is a push to get up to five doses out of a single dose, by injecting the vaccine differently.
"You know the saying - if you don't know where you're going, any road will get you there? Some days, that's happening," said Michael Osterholm, epidemiologist at the University of Minnesota.
The vaccine was originally developed to protect against smallpox, in case that disease ever returns. But scientists are confident that it will provide at least some degree of protection against the similar monkeypox virus.
Jynneos's regulatory authorisations are largely based on animal experiments with monkeypox, as well as data showing that it is safe and prompts an immune response in humans.
In total, only 40 per cent of countries experiencing monkeypox outbreaks currently have access to the vaccine, according to a WHO spokesperson.
Brazil and Peru, for example, among the worst-hit countries globally, have no vaccines yet, although negotiations are under way and deliveries are due later this year. The WHO is trying to set up an equitable distribution mechanism, but has no access to vaccines.
"You can't allocate doses you don't have," said Nicole Lurie, director of preparedness at the Coalition for Epidemic Preparedness Innovations (CEPI), who has been involved in the discussions.
Until more supply and data on efficacy are available, governments should prioritise the vaccine for the most vulnerable, such as immunocompromised men who have sex with multiple male partners, some disease experts said.
Scientists at Yale University estimate that fewer than half high-risk men need to be vaccinated to bring the outbreak under control, depending on the infectiousness of monkeypox, the effectiveness of contact tracing and the efficacy of the vaccine - all unknowns.
"We are really in a race against the virus," said Melanie Chitwood, co-author of the study at the Yale School of Public Health.
"What is troubling about the vaccine shortage globally – the longer it takes us to reach the critical threshold of vaccinated individuals, the more time the virus has to do its work."