CDC offers explanations for mild flu season

The U.S. Centers for Disease Control and Prevention recently offered several explanations for this year's mild influenza season in the United States.

The CDC said that mild weather, high vaccination rates and high population immunity owing to a fairly stable set of circulating viruses may be behind the lower than normal flu activity, according to CIDRAP News.

Flu season officially began, as measured by the percentage of positive tests on respiratory specimens, two weeks ago, but the CDC's weekly flu update shows that flu activity remains low when measured by the number of outpatient medical visits for influenza-like illnesses. Only one state has reported high ILI activity as of last week.

"This is the latest start to the season in 29 years," Dr. Joseph Bresee said at a CDC press briefing, CIDRAP News reports.

Bresee said that all three flu subtypes that were included in the seasonal vaccine are circulating, but that H3N2 viruses have made up more than 70 percent of the isolates identified. There have been relatively few type B viruses isolated.

"The timing of influenza and how severe it is each year is highly variable and hard to predict," Bresee said, according to CIDRAP News. "One factor is that the viruses circulating this year are quite similar to the viruses last year, so possibly there is high immunity to the viruses in the population.

"We've seen vaccine coverage rates continue to trend upward, which should lead to less disease over time."

Bresee also said that the unusually mild winter may also be a factor. Colder weather generally leads to flu viruses remaining viable for longer periods and people tend to cluster together inside more where viruses can spread.

The CDC expects flu indicators to increase in the coming weeks and considers this year's pattern to be unusual but not unprecedented.