USA - Recent results from studies with H1N1 influenza A virus

Limited cross-reactivity of serum samples from pigs infected U.S swine influenza viruses or vaccinated with commercial vaccines was demonstrated against the new S/O H1N1 influenza virus (A/CA/04/2009) as measured by the standard HI test. The results of this experiment suggest that pre-existing immunity induced by swine influenza viruses circulating in the U.S may not protect pigs against the new S/O H1N1 influenza virus presently circulating in people. Importantly, vaccines currently used to protect pigs in U.S swine operations against swine influenza virus may not be effective against the new S/O H1N1 influenza virus.