Sick of Swine Flu? Here Comes H3N2

With reporting by Martin Enserink.

Although the world’s attention is focused on the novel H1N1 virus causing the swine flu pandemic, H3N2, a seasonal strain of influenza, has popped up in many East Asian countries—and some variants in circulation may outfox the seasonal vaccine in use. “We have seen that H3N2 viruses have been in fairly broad circulation in some of the countries there,” Keiji Fukuda, special adviser on pandemic to the director-general of the World Health Organization, said at a press conference today.

The H3N2 strain is one of three in the seasonal influenza vaccines. But if the H3N2 strain in circulation differs substantially from the one used to make the vaccine, the vaccine may offer less protection, and more people will get sick than usual. “For the current H3N2, we don't have such studies, so I can't tell you right now the degree the current seasonal vaccine will protect against the H3N2 virus,” Fukuda says.

However, some early indicators from China suggest that the main H3N2 in circulation there may be a mismatch with the vaccine strain. The H3N2 used in the seasonal vaccine is what’s known as the Brisbane subtype. Yuelong Shu, director of the Chinese National Influenza Center in Beijing, told ScienceInsider in an e-mail that, according to their studies of recent isolates there, the “majority of the H3N2 virus is Perth-like virus.” Shu noted that the center has no data to estimate the efficacy of the seasonal vaccine against this subtype.

The novel H1N1 virus accounts for more than 99% of the isolates subtyped since 30 August in the United States, Nancy Cox, head of the influenza division at the U.S. Centers for Disease Control and Prevention told ScienceInsider in an e-mail 10 days ago. But she said that 17% of the H3N2 they had subtyped was Perth-like.

Typically, H3N2 moves from East Asia to Europe and the United States, but Shu said it is too soon to know whether the Perth-like subtype in circulation will make major inroads in the West or reduce the effectiveness of the seasonal vaccine. “I personally think it is too simple to [draw] this conclusion,” cautioned Shu. “We may need more data.”