Studies on Influenza Virus Transmission between Ferrets: the Public Health Risks Revisited

Fouchier, Ron A. M.

The vaccination of laboratory personnel against the homologous A/H5N1 virus under investigation produces another layer of safety that results in further risk mitigation. Given the generally accepted efficacy of influenza vaccine of~65% for laboratoryconfirmed influenza in healthy adults (18) , vaccination reduces the probability of an LAI that results in viral escape to below 0.35 ϫ (1.4 ϫ 10 Ϫ6 ), or Ͻ5 ϫ 10 Ϫ7 per person-year. The 65% value is almost certainly an underestimate, because this number is taken from general population studies that include individuals with impaired immunity and nonexact matches between the vaccine antigen and the circulating viruses. It is important to note that the antibody titers in our vaccinated laboratory workers are high (geometric mean titer, 987; range, 160 to 10,240) compared to the titers generally accepted as protective against seasonal influenza (Ն40) (19) and that individuals are revaccinated if and when their antibody titers decrease (12) .