Merck, which provides around 50 per cent of US Hib vaccine supplies, initiated the voluntary recall on Wednesday after routine inspections of it manufacturing process revealed a possible source of contamination.
The company discovered a problem with its production equipment that created the potential for micro-organisms to survive the sterilisation process that forms part of the vaccine's manufacture.
The recall affects 11 lots of the company's Pedvaxhib vaccine, which protects infants against infections caused by the Haemophilius influenzae type b bacteria (Hib), and a further two lots of the firm's Comvax combination vaccine which protects infants from the Hib infection and hepatitis B.
The potentially contaminated doses were distributed from April this year, but according to the US Centers For Disease Control (CDC) and Food and Drug Administration (FDA) there have thus far been no reported cases of injury or adverse effects associated with the vaccine.
"The potential for contamination of any individual vaccine is low," Merck said in a statement issued on Wednesday.
"And, if present, the level of contamination would be low."
Merck generally supplies approximately half of the US's annual 14 million dose requirement of Hib vaccine, with the remainder provided by Sanofi-Pasteur, the vaccine arm of French firm Sanofi-Aventis.
"The 50/50 split that we've seen in the past is not a fixed amount," said Anne Schuchat, director of the CDC's Center for Immunization and Respiratory Diseases.
"We've been working with Sanofi to work on increasing their production while Merck is working on the problem that they have had.
"As far as we understand it, we cannot set a date after which Merck's production will be back in force, and we do not know…exactly how many doses Sanofi will be able to produce."
Not only is there some uncertainty regarding when stocks of the vaccine will be back to standard levels, but the CDC's back-up stockpile has also been affected by the recall, with the organisation's ability to minimise the impact of the inevitable shortfall therefore substantially compromised.
Over the coming days a team of experts on public health and paediatrics will meet with representatives of the vaccine community and the CDC advisory board to review the situation regarding current vaccine supplies, and try to determine whether temporary changes should be made to the nation's vaccine recommendations.
In the meantime, physicians are being advised not to administer vaccine from the recalled lots.
Patients who have already received a shot from the highlighted batches should continue their immunisation programme with vaccine unaffected by the recall, but will not need to be revaccinated to replace the initial vaccine as efficacy was not affected.
Double whammy The Hib vaccine recall came hot on the heels of bad news for another of Merck's products.
Earlier this week the firm's Canadian subsidiary, Merck Frosst, reported that Canadian health authorities had suspended from use three lots of the company's measles, mumps and rubella vaccine, M-M-R II.
Health Canada decided on the move after five suspected cases of anaphylaxis were reported in young adults with a previous history of allergy given the treatment.
The affected patients were given the shot as part of a catch-up vaccination campaign following a mumps outbreak, and all recovered following treatment.
Authorities nonetheless decided to temporarily suspend three lots of the M-M-R II vaccine with 200,000 doses affected by the decision.
Health Canada has requested that Merck Frosst Canada provide a written report on any manufacturing or safety problems associated with the lot that caused the most concern - lot 1529U - or any of its bulk components.
While this temporary suspension might not have a huge impact on the firm (presuming no inherent problems in the manufacturing process are identified) the possible contamination of the Hib and Hib/hep B combination vaccines could potentially cause more of an issue for Merck.
While the general pattern of supply of these vaccines to the US in the past has tended to be a 50/50 split between Merck and Sanofi Pasteur, there is a possibility that Merck's moves to recall could jeopardise their share of their pie with Sanofi now set to step in and up production to deal with the vaccine shortfall.
Whether this could see Sanofi playing a more prominent role in Hib vaccine supply to the US in the long-term and the 50/50 division tipping more in the French firm's favour, will presumably depend on how quickly Merck can sort out its manufacturing issues and get back in the game to secure its share of the market.
Paediatric vaccines generated $428m in sales for Merck over Q3 2007.