Reply to “Studies on Influenza Virus Transmission between Ferrets: the Public Health Risks Revisited”

Lipsitch, Marc; Inglesby, Thomas V.

Fouchier further argues that the risk of onward transmission from an LAI would be reduced relative to those we posited, a range of 5 to 60%. He suggests that prophylaxis and vaccination should not only reduce the probability of infection (discussed above) but also reduce the probability of onward transmission by a factor of 100. The effect of prophylaxis and vaccination should be accounted for, as we noted in our article (5), but again only if the infection occurs in the source laboratory where workers are prepared; moreover, the factor of 100 reduction is much too optimistic, for the following reasons. (i) It assumes the infection is detected, which may or may not occur before spread. (ii) It assumes that vaccines and antivirals are given and effective against the labengineered virus, which is not guaranteed or in some cases even likely for the reasons noted above. (iii) If infection is detected before spread, and if vaccines and antivirals are given to the exposed person rapidly, and if the strain is susceptible to the antiviral, the reduction in infectiousness of a vaccinated, antiviraltreated case is probably closer to a factor of 5 to 8 than a factor of 100. This estimate is based on an assumption of multiplicative effects of antivirals and vaccination, using clinical data to estimate that oseltamivir reduces infectiousness approximately 5Ï« (by 80%) (21) and a meta-analysis of published studies showing no reduction in infectiousness (22) yet suggesting a "best guess" of a 1.7-fold (40%) reduction in infectiousness from a well-matched inactivated vaccine (22) . If infection is not detected before spread, or if vaccines and antivirals are not given to the infected person(s) rapidly enough to prevent spread, or if the strain is not susceptible to the antiviral or vaccine, then there is no reduction in risk.