US flu levels low as new season's reporting starts

In its first surveillance report for the new US flu season, overall activity is low, with H3N2 as the dominant strain, the US Centers for Disease Control and Prevention (CDC) said today.

It urged people to get vaccinated ahead of an uptick in flu, which the CDC commonly sees in October. Protection typically kicks in about 2 weeks after immunization. The season usually peaks sometime between December and February.

Institutional outbreaks reported

Flu markers, such as percentages of clinic visits for flu and respiratory specimens that test positive for flu, are still well below their baselines, the CDC said in its FluView report. Last week the percentage of people visiting clinics for flulike illness increased slightly compared with the week before, from 1.1% to 1.2%.

Most states are reporting minimal flulike illness activity, with Georgia the only one reporting that indicator as low. Guam is the only location reporting widespread geographic spread. Hawaii is reporting regional spread, and Oklahoma is reporting local spread.

In a Twitter post on Oct 14, the CDC said that though flu activity remained low, it had received reports of early institutional outbreaks across the country.

The overall percentage of specimens testing positive for flu was 1.2% last week, with the number varying regionally from 0.3% to 2.8%.

H3N2 strain most common

Influenza A accounted for 93% of identified flu viruses, with 92% of the subtyped influenza A viruses the H3N2 strain. Of flu viruses characterized between late May and late September, the few 2009 H1N1 viruses that were identified were antigenically similar to the corresponding strain in this season's vaccine.

Many experts are closely watching the H3N2 strain, which caused problems during last flu season due to a mismatch between the circulating and vaccine strains and is increasingly seen as one of the weak links in flu vaccine effectiveness. Also, an expert group from the World Health Organization (WHO) that recommended vaccine viruses for the Southern Hemisphere's 2016 flu season changed its H3N2 recommendation to a strain (A/Hong Hong/4801/2014) not reflected in the Northern Hemisphere's vaccine for the current season.

The CDC said genetic sequencing through Sep on 149 H3N2 viruses found that all viruses were part of genetic groups for which a majority of antigenically characterized viruses were similar to A/Switzerland/9715293/2013, a component of the Northern Hemisphere's vaccine. It added that antigenic characterization of a subset of 58 viruses revealed that all were A/Switzerland/9715293/2013.

So far the CDC has seen no significant drift in currently circulating flu viruses, as it did last season.

Flu supply status, FluMist delay

The agency predicts companies will make 171 to 179 million flu vaccine doses for the US market. As of Oct 2, distributors had shipped about 98.2 million doses to providers, according to updated information yesterday from the CDC.

Flu vaccine formulations continue to evolve this season, with quadrivalent vaccines that protect against both influenza B lineages making up a larger part of the US supply. Intramuscular vaccines are now available in both trivalent and quadrivalent forms, and all nasal and intradermal doses will be quadrivalent this year, according to background information from the CDC. Also a jet injector can be used with one trivalent vaccine to immunize adults through age 64.

MedImmune has encountered some unforeseen challenges with the production of FluMist (also known as live attenuated influenza vaccine, or LAIV) that have delayed deliveries, Melissa Garcia, a company spokeswoman, told CIDRAP News. She said the problems involve two of the four strains in the vaccine and will result in a substantial portion of FluMist being delayed compared with previous seasons.

She said that so far nearly 5 million FluMist doses have been distributed, and a significant number of additional doses are expected through the end of the year. "The CDC supports vaccinating as long as flu viruses are circulating. We continue to support vaccination as soon as product is available and throughout the season," she noted.

See also:

Oct 16 CDC FluView report

CDC Flu Twitter feed

Oct 7 CIDRAP News scan "Conference data show low H3N2 vaccine effectiveness"

Sep 24 CIDRAP News story "WHO panel changes 2 strains in Southern Hemisphere flu vaccine"

CDC flu vaccine supply page

CDC background on 2015-2016 flu season

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