US hunt for novel flu reveals persistence of seasonal strains

Jun 16, 2009 (CIDRAP News) – One byproduct of the pandemic of novel H1N1 influenza is increased evidence of the extent to which "seasonal" flu viruses stick around in the summertime.

Because of the novel virus, clinicians are testing many more patients for flu than they normally do at this time of year, according to federal health officials. Most are testing positive for the new virus, but some are turning out to have seasonal flu strains, even though the normal flu season ended weeks ago.

In the week that ended Jun 6, about 1.8% (49 of 2,681) of the flu viruses identified in lab tests reported to the Centers for Disease Control and Prevention (CDC) were seasonal viruses—either influenza A/H1N1, A/H3N2, or B, according to the CDC's latest weekly flu surveillance report.

The agency says that about 89% of the isolates were the novel virus, while the rest were not subtyped. (The 89% includes 315 isolates that could not be subtyped by reference laboratories; the CDC says its own testing of those isolates nearly always shows them to be the novel virus.)

Clinicians who see patients with respiratory illnesses at this time of year usually don't test them for flu, because normally it has faded by now, said Dr. Anthony Fiore, a medical epidemiologist with the CDC's influenza branch.

"Now people who don't normally look for flu are looking more than they ever have," Fiore said. "We're seeing that some of the respiratory illnesses that occur even late in the season may be due to seasonal flu viruses that we didn't appreciate in the past."

Fiore added that there has long been awareness that flu viruses continue to circulate at a low level in the summer, but the current level of testing is shedding more light on the situation.

A CDC flu surveillance report issued last October said that just 179 flu viruses were found among 25,031 respiratory specimens tested through all of last summer (from mid-May through September). In comparison, the CDC's latest surveillance report shows that 49 seasonal flu isolates were identified just in the week that ended Jun 6.

Fiore said some summer cases of flu in the United States probably are related to travel to the southern hemisphere, which has its winter flu season during the northern summer.

He said it's not clear whether the level of seasonal flu being detected now is unusual, because this level of surveillance has not been conducted at this time of year before. "We don't know whether that's how it was last year," he said.

He added that the current findings of seasonal viruses could be related to the fact that the flu season didn't peak this year until late February, considerably later than usual.

"I guess we don't know that anything different is occurring right now with seasonal flu viruses. We don't have reason to think that," Fiore said. It's more likely that "this is an artifact of clinicians looking very hard for flu viruses at a time they don't normally look."

The CDC normally stops issuing its weekly flu surveillance report in late May before resuming in the fall, he noted. But with the novel H1N1 virus circulating in much of the nation, the agency probably will keep publishing the updates all through the summer.

Three deaths of children from flu-related causes were cited in last week's report—two of them from the novel H1N1 virus and one from the seasonal H1N1 virus. The deaths occurred between Feb 15 and May 30 and brought the total of pediatric flu-related deaths since last September to 70.

With the novel virus circulating, flu was widespread in eight states (Arizona, Connecticut, Delaware, Massachusetts, New Jersey, New York, Pennsylvania, and Rhode Island) in the first week of June, the CDC report said. Nine states reported regional flu activity, and most of the rest reported either localized outbreaks or sporadic cases.

Overall, 1.9% of visits to the CDC's sentinel medical providers were attributed to flu-like illnesses, which was below the national baseline of 2.4%. But the proportion was above regional baselines in two regions, covering New England, New York, New Jersey, Puerto Rico, and the US Virgin Islands.

See also:

CDC flu surveillance report for the week that ended Jun 6

CDC flu report for the week that ended Oct 4, 2008

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