Prior seasonal influenza infections protective against 2009 Pandemic flu (if you're a ferret)

Looking back at the 2009 flu season, I realize how calm it is now compared to then.  I hope it stays that way!  In that pandemic, the attack rate was 50% in young populations compared to perhaps 10% in adults.  Why would this be the case? Serologic analysis showed little cross-reactivity between recent seasonal influenza A(H1N1) viruses and pandemic A(H1N1). The authors of a recent JID paper postulated that the lower attack rate in adults could result from multiple past exposures to viruses with similar B epitopes or since there is conservation of T cell epitopes between pandemic H1N1 and seasonal influenza A, then cellular immunity may also reduce disease severity. There's also the fact that adults responded to a single dose of pandemic H1N1 vaccine, while children did not, suggesting that past exposure to seasonal strains is important.

To examine the role of prior immune reponses in seasonal influenza on exposure to pandemic H1N1, Laurie et al. in the Oct 1 JID, studied the impact of one or two prior infections in a ferret model.  The found that a single prior infection with a seasonal influenza A virus, A/Fukushima/141/2006 (H1N1) or A/Panama/2007/1999 (H3N2), reduced the duration of shedding following challenge with 2009 pandemic H1N1, but not reduce the infection rate nor did it reduce the transmission to other ferrets.

The authors then tested whether two prior infections with seasonal influenza was protective against pandemic H1N1.  They determined that infection with seasonal A(H1N1) followed by A(H3N2) reduced the infection rate along with the amount and duration of shedding in ferrets challenged with pandemic A(H1N1). Interestingly, no virus was transmitted to other ferrets, nor did the exposed naïve ferrets experience seroconversion to pandemic flu.

Good news, if you're a ferret.

Laurie KL, et al JID, October 1, 2010.