Image copyright EPA Image caption A man waits to get checked for COVIVENT
The US Centers for Disease Control and Prevention says a new round of immunisation is needed after an unprecedented breach involving a back-up vaccine supply in April, 2018.
Last year, health chiefs admitted that two doses of the anti-coagulant COVIVENT, supplied by Chiron, were accidentally swapped.
The vaccine is used by immunocompromised patients, including some immunosuppressed adults.
A new series of boosters is on the way and should arrive in July.
Speaking at a news conference, Dr Anne Schuchat, the CDC’s acting director, said: “These are all preparations we have. These aren’t bottles of vaccine and maybe we need to start reprinting those out.
“Certainly, we know for many people, so many individuals immunosuppressed by chemotherapy or by HIV, all of that requires that they take multiple doses of the flu vaccine.”
Image copyright COVIVENT Image caption Novartis, the vaccine supplier, was responsible for the second batch of the vaccine
The CDC has traced the accidental breach to Novartis, the vaccine manufacturer.
The first batch of vaccines has since been destroyed but last November the manufacturer was only able to deliver “approximately three-quarters” of the fresh vaccine – about two doses – which was used for the second round of immunisation.
Officials have stressed that there is no evidence that anyone fell ill after taking part in the immunisation programme.
A review into the incident in April revealed that the virus strain, Influenza A H3N2, may have been missed during clinical trials at Novartis.
While the second batch of COVIVENT vaccine was installed in batches 20 and 21 – which were used for a week in the summer months – there was no evidence that this was a deliberate cover-up.
However, Dr Schuchat said: “Because our tracking system is not in place, we don’t know if it happened simultaneously.”