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

Fouchier, Ron A. M.

Initial calculations of the potential risks associated with research on influenza virus transmission via respiratory droplets or aerosols between ferrets (1-4) used reports on select agent theft, loss, and release collected by the U.S. Centers for Disease Control and Prevention (CDC) from 2004 to 2010 (7) to calculate the probability of occurrence of LAIs. Although these reports have limitations (1, 4, 7) , they provide the most recent account of LAIs in the United States and probably reflect the current state of the art in biosafety and biosecurity practices better than older studies on laboratory incidents (8, 9) , e.g., as a consequence of the implementation of the U.S. select agent program and best practices developed in biosafety and biosecurity in general over the last decades. From 2004 to 2010, 11 LAIs in total were reported to the U.S. CDC, 4 of which occurred in biosafety level 3 (BSL3) facilities. During this 7-year period, on average 10,000 individuals per year had access to select agents in an average of 292 laboratories per year, thus totaling 2,044 laboratory-years and 70,000 personyears of follow-up (7) . From these data, the probability of occurrence of LAIs under BSL3 conditions was calculated as 4/2,044 (or 2 ϫ 10 Ϫ3 ) per laboratory-year, or 4/70,000 (or 5.7 ϫ 10 Ϫ5 ) per person-year (1) (2) (3) (4) . These estimates, however, do not take into account specific pathogen types or research settings. This is crucial, because working practices in, e.g., virology and microbiology laboratories are different and because each biosafety laboratory is unique (10, 11) . Research facilities and the experiments that are conducted are therefore appraised through targeted risk assessments, in which the planned studies are scrutinized before any experiment is started. On this note, it is important that none of the LAIs reported to the U.S. CDC from 2004 to 2010 involved viruses (7) , and the risks of LAIs associated with work on viral pathogens should thus be estimated as less than 1 per 2,044 (Ͻ5 ϫ 10 Ϫ4 per laboratory-year), or less than 1 per 70,000 (Ͻ1.4 ϫ 10 Ϫ5 per person-year). Unfortunately, the report by Henkel et al. (7) does not specify how many of the 2,044 laboratory-years and 70,000 person-years were related to BSL3 facilities versus BSL2 and BSL4 facilities. Thus, using 2,044 and 70,000 as the denominators yields an underestimation of the true probability of LAIs under BSL3 conditions, as discussed previously (1, 4) .