The firm, a unit of AstraZeneca, recommended that vaccines with an expiration date between January 19 and January 26, 2010 should not be used, but stressed that the doses are safe and that people who have been inoculated need not seek revaccination.
Company spokesman Tor Constantino told Bloomberg that the 13 FluMist lots covered by the recall, which comprise some 4.7m doses, were distributed in October and November and that the “vast majority” have already been used.
Norman Baylor, director of the office of vaccines research and review at the Food Drug Administration (FDA), estimated in Reuters interview that only about 3,000 doses of FluMist from the affected lots are thought to remain.
Maryland-based MedImmune is the second firm to recall an H1N1 vaccine in as many weeks behind French anti-infectives specialist Sanofi Pasteur, which withdrew 800,000 doses of its product last Tuesday after detecting a similar drop in potency.
At the time, US Centers for Disease Control and Prevention (CDC) immunization head Anne Schuchat suggested Sanofi’s vaccines may have lost potency because, over time, the viral antigens they use to stimulate an immune response may adhere to the sides of syringes used to deliver them.
MedImmune is yet to determine why the potency of its product dropped, although given that FluMist relies on live attenuated virus and nasal delivery rather than injection, the cause is unlikely to be the same as Sanofi’ product.
Last week, the CDC reports that over 60m US citizens have received one dose an H1N1 vaccine, which is equivalent to half of the country’s stockpile.