Study leader Ram Sasisekharan of the Massachusetts Institute of Technology said in the past 100 years, influenza viruses that emerged from pigs or birds have caused several notable flu pandemics. When one of these avian or swine viruses gains the ability to infect humans, it can often evade the immune system, which is primed to recognize only strains that commonly infect humans.
In the study, the researchers compared the 1968 H3N2 strain and about 1,100 H3 strains now circulating in pigs and birds, focusing on the gene that codes for the viral hemagglutinin protein.
After comparing hemagglutinin genetic sequences in five key locations that control the viruses' interactions with infected hosts, the researchers calculated an "antigenic index" for each strain.
Seeking viruses with an antigenic index of at least 49 percent and glycan-attachment patterns identical to those of the 1968 virus, the research team identified 581 H3 viruses isolated since 2000 that could potentially cause a pandemic. Of these, 549 came from birds and 32 from pigs.
The researchers then exposed some of these strains to antibodies provoked by the current H3 seasonal-flu vaccines. As they predicted, these antibodies were unable to recognize or attack these H3 strains.
The study, published in the journal Scientific Reports, found of the 581 hemagglutinin sequences, six swine strains already contain the standard hemagglutinin mutations necessary for human adaptation, and are thus capable of entering the human population either directly or via genetic reassortment.