By MITZI BAKER

"It was very confusing to hear about this in the mass media, especially when they’re throwing out all these ‘H3N2s’ and ‘H1N5s,’ " said Paul Eckburg, MD, a postdoctoral scholar, who gave an infectious disease grand rounds talk on the 2003-04 flu season last week.

To clear up some of the confusion, Eckburg provided an overview of the flu virus and the infection it causes, including what the H’s and N’s mean and the story behind the "wrong" vaccine. He also discussed how the early peak of flu cases in December highlighted the need to prepare for a potential pandemic on par with that in 1917-18 when 20 million died worldwide.

The bottom line, said Eckburg, is that experts can’t predict what will happen for the rest of the season, but "we’re on the down-slide. There are clearly less influenza and influenza-like illnesses now.

"The interest lies in the fact that the vaccine is not covering the strain that is circulating, so more people are getting symptoms," he said. "But the symptoms they have seem to be on the mild side. We’re lucky this season because there are no data to suggest that this year’s virus is more virulent than the virus seen in past years."

Flu vaccines are designed to stimulate an antibody response against markers on the surface of the virus to reduce the likelihood of infection and the severity of disease if infection occurs, Eckburg said. Two types of influenza virus cause significant human disease: type A and type B, and vaccines typically include either killed or weakened virus of both types. Influenza A virus is further categorized into subtypes based on its version of two surface proteins: hemagglutinin (H) and neuraminidase (N).

"Type A H3N2 is a very common strain that circulates nearly every year," said Eckburg. "The confusing thing is that there are many types of H3N2 viruses." This year, he said, the experts designed a vaccine that included an H3N2 strain called Panama, but one called Fujian emerged instead as the major culprit. Antibodies against one influenza type give limited or no protection against another.

Additionally, he said, this year is unusual because there is almost no strain B circulating. Last year, about half of flu cases were caused by type A and half by type B.

In the United States, the flu season can range from November through March or April. In the past 21 flu seasons, the heaviest activity occurred most often in February, but in four of those years, activity peaked in December, as it appears to have done this season, Eckburg said.

Vaccine offsets doozy of flu season (12/10/03)

Flu shots cost-effective for healthy, younger adults (8/21/02)