The UK is waiting to receive around 100,000 doses of smallpox vaccine amid reports the country is set to run out.
The UK Health Security Agency (UKHSA) said 150,000 doses have been procured, but only 50,000 have been received so far due to supply issues.
There are only around 5,000 doses of the jab left, Sky News reported.
The remaining stock is set to arrive in September.
The NHS announced late last month that it was stepping up its vaccination programme against monkeypox in London as more supplies of a jab became available.
Dr Mary Ramsay of the UKHSA said: “UKHSA moved early to procure over 150,000 doses of the smallpox vaccine from the global manufacturer.
“Around 50,000 of these have been received to date and made available to the NHS for distribution across the country.
“The rollout is continuing at pace, with the vaccine being offered to individuals at higher risk of coming into contact with monkeypox in order to offer them protection and to help contain the current outbreak.
“The remaining approximately 100,000 doses is expected to arrive in the UK in September.
“The thousands of vaccines administered by the NHS to date among those at highest risk of exposure should have a significant impact on the transmission of the virus.”
Thousands of cases of the illness have been recorded in the UK, with the majority transmitted between gay and bisexual men, and men who have sex with other men.
Vaccination experts have recommended that gay and bisexual men be offered the smallpox jab Imvanex.
Ian Green, chief executive of sexual health charity the Terrence Higgins Trust, said: “There needs to be urgent action to salvage this situation. We know many gay and bisexual men will be worried by news of vaccine supplies running low and our advice to them is please do not panic.
“It’s crucial we have transparency from the UKHSA about what’s happened and when we can expect more doses to reach the country, as the vaccine programme is absolutely crucial to getting on top of the monekypox outbreak.
“The very limited doses of vaccine we have left in the country must now be prioritised for those most at risk in the places with the highest reported cases.
“That means ensuring sexual health services in places like Brighton, Manchester and Essex are given provision alongside London.
“It’s high time we got a proper grip on the monkeypox response and for that we need to see this taken seriously at the highest level of Government.”
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