Flu continues late-season surge, claims more children

US influenza activity continued a late-season surge in the second week of March, with widespread circulation cited in 40 states and another eight children reported as victims of the virus, the Centers for Disease Control and Prevention (CDC) reported in its latest flu update.

Geographically widespread flu activity was reported in 40 states plus Puerto Rico, up from 37 states and Puerto Rico a week earlier, according to the Mar 18 update, which covers the week of Mar 6 to 12.

The count of eight flu-related deaths in children was four times as high as the previous week and raised the season's total to 28 deaths. Although the season isn't over, the death toll at this point is far below the cumulative pediatric death tolls in the three previous seasons, all of which were well over 100.

High ILI in 14 states

The CDC reported yet another increase in influenza-like illness (ILI) cases, with 14 states, New York City, and Puerto Rico reporting high ILI counts, as compared with 10 states and Puerto Rico a week earlier. The number of states with moderate ILI levels stayed the same, at 13.

A closely related marker, the estimated share of clinic visits prompted by ILI, was 3.7%, up from 3.5% the previous week. It stayed well above the 2.1% national baseline.

Another number that increased again was the proportion of respiratory samples that tested positive for flu: 23.1% of 25,855 specimens, versus 20.6% of 23,910 specimens a week earlier.

On the other hand, the share of deaths attributed to pneumonia and flu declined very slightly, according to the CDC's 122 Cities Mortality Reporting System. The number was 6.9%, down from 7.0% the week before, and was below the 7.2% epidemic threshold.

The CDC's other surveillance system for pneumonia and flu deaths, operated by the National Center for Health Statistics, put the number at 7.5%, but that covered an earlier time interval, the week that ended Feb 27.

As for flu-related hospitalizations, the cumulative estimate for the season reached 14.5 per 100,000 people, up from 10.4 per 100,000 a week earlier. Elderly people (65 and older) accounted for the largest share, with a rate of 37.2, up from 27.6 the week before. They were followed by the 50-to-64 age-group (21.3) and children under age 5 (20.9).

H1N1 continues to predominate

Although the last few weeks of flu season often bring an increase in influenza B cases, that doesn't seem to be happening this year. Of close to 6,000 viruses that were typed by laboratories for the week, 76.4% were type A and 23.6% were type B. For the whole season so far, the split is 74.0% type A and 26.0% type B.

Among type A viruses, 2009 H1N1 isolates continue to greatly outnumber H3N2 isolates, accounting for 79.9% of subtyped viruses for the week and 78.3% for the season, the CDC reported.

See also:

CDC FluView report

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