As flu activity tails off, attention turns to Southern Hemisphere

Lisa Schnirring

May 25, 2012 (CIDRAP News) – Influenza is approaching summertime levels in most parts of the United States, though virus activity is expected to persist over the coming weeks, the US Centers for Disease Control and Prevention (CDC) said today in its last full report of the season.

The percentage of respiratory specimens that tested positive for flu increased slightly to 13.9% last week, compared with 13.3% the week before, the agency said. Other flu markers remained below their baselines.

Two pediatric flu deaths were reported, one from H3N2 and one from influenza B, raising the season's total to 26. New York was the only state that reported wide geographic flu activity. Alabama was the lone state to report low flu activity, with the rest reporting minimal activity.

Flu season is also nearing its end in Europe, with all countries except for Slovakia reporting low intensity flu activity, according to an update today from the European Centre for Disease Prevention and Control (ECDC).

Testing of 119 sentinel specimens from 22 countries found that 6.7% were positive for flu. The ECDC said that, with the season ending, it will switch to fortnightly flu updates.

Globally, the flu season is coming to an end for most Northern Hemisphere countries, the World Health Organization (WHO) said today.

In the Southern Hemisphere, where the flu season typically begins around May, flu activity is at low or at interseasonal levels, with some very small increases detected in Chile and some activity persisting in sub-Saharan Africa.

Earlier in the season, most viruses were a close antigenic match to those in the trivalent vaccine, but by midseason divergence in H3N2 viruses was found in both the United States and Europe, the WHO said.

In February, WHO advisors recommended changing H3N2 and influenza B components for the Northern Hemisphere's flu vaccine. However, the group weighed in on the Southern Hemisphere's vaccine in October 2011—before the changes were detected—so the recommendation was for the same three strains as last season.