Late start to Northern Hemisphere flu season feels strong H1N1 impact

After a slow start to the flu season in many Northern Hemisphere countries, activity is starting to pick up, much of it led by the 2009 H1N1 virus, known to cause severe illnesses even in younger healthy people, the World Health Organization (WHO) said yesterday in a pair of updates.

The WHO also addressed genetic changes in the former pandemic strain, but said so far evidence suggests the circulating strains are similar enough to the one in the vaccine to trigger protection.

Activity high in some locations

Parts of the Middle East have been the main hot spots this winter, but activity may have peaked in locations such as Iran, Jordan, and Oman, the WHO said. However, flu activity is climbing in Europe, especially northern and eastern countries where flulike illnesses and severe infections from 2009 H1N1 are up sharply.

Finland, Russia, and Ukraine are reporting very high flu activity, with Belarus, Greece, Ireland, and Malta reporting high-intensity levels, according to the WHO.

Severe respiratory illness surveillance systems in several European countries are reporting increasing numbers of cases linked to 2009 H1N1. Likewise, systems on the continent that track lab-confirmed flu at hospitals and intensive care units (ICUs) are reporting 2009 H1N1 virus involvement in the majority of cases, mainly in people ages 15 to 64 years old.

Japan and South Korea are reporting increasing flu activity, with the 2009 H1N1 virus predominating, and North America is seeing a similar rise, though overall levels are still low, the WHO said.

However, the mix of flu strains varies in some countries, such as northern China where H3N2 and influenza B are predominating. The WHO said a few European countries are reporting cocirculation of H3N2 and influenza B.

Flu levels are low in in tropical Africa, tropical Asia, and the Southern Hemisphere, but some parts of the tropical Americas are reporting some spikes in activity, including Puerto Rico, Guadeloupe, and Costa Rica.

CDC getting reports of severe flu

In a related development, the US Centers for Disease Control and Prevention (CDC) said today that flu activity is increasing in the United States and that it has received several reports of severe flu infections. Though all three strains have been circulating since October, the recent reports of serious illness in young- to middle-aged adults have been linked to the 2009 H1N1 virus.

The CDC announced a clinician's call for Feb 16 to update health providers on flu activity in the United States and to share its latest vaccination and treatment recommendations.

The last time the country experienced a heavy 2009 H1N1 burden year was during the 2013-14 flu season.

Eyeing H1N1 for clues about vaccine protection

In a separate risk assessment of global 2009 H1N1 activity, the WHO said hospitalization and ICU levels countries are reporting seem to be similar to past years when the virus dominated and when young and middle-aged adults were hit with severe disease.

An analysis of 2009 H1N1 viruses circulating since September in more than 30 countries, including those reporting severe infections, show that all are in the 6B genetic subgroup, in which a subset of viruses with the same amino acid changes has emerged, according to the WHO. However, despite the virus changes, most—including the ones with the amino acid changes—are still antigenically similar the vaccine virus.

The agency added that tests with recent 2009 H1N1 viruses at the US Centers for Disease Control and Prevention (CDC) showed that they were well inhibited by post-vaccination serum collected from healthy adults last flu season.

Yesterday the European Centre for Disease Prevention and Control (ECDC) addressed rises in 2009 H1N1 activity in its flu season update. It said it was encouraged by a report about modest flu vaccine effectiveness based on results from New Zealand's 2015 season, but the ECDC said it's not clear yet what impact the genetic changes might have.

The WHO said the findings hint that the H1N1 component included in the current Northern Hemisphere flu vaccine should provide good protection, and it strongly recommended that people in high-risk group be vaccinated against flu.

Few signs of antiviral resistance

So far only 2 of 450 H1N1 virus tested for antiviral resistance have shown reduce susceptibility to neuraminidase inhibitors, so currently circulating strains should be sensitive to oseltamivir (Tamiflu) and zanamivir (Relenza), which the WHO said should be administered early to those at increased risk for flu complications.

The WHO's flu vaccine advisory group meets Feb 22 through 25 to discuss the circulating strains and to recommend the ones to include in the Northern Hemisphere's 2016-17 flu vaccine.

See also:

Feb 8 WHO global flu update

Feb 8 WHO 2009 H1N1 risk assessment

CDC COCA call information

Feb 8 CIDRAP News scan "H1N1 and influenza B making marks on Europe's flu season"

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