Flu Shot Effectiveness Numbers Out for Current Season

Overall vaccine effectiveness (VE) for the flu shot is 47% for the current season, with 46% effectiveness against this year’s dominant strain, influenza A(H1N1)pdm09, researchers found.

Adjusted VE against overall medically-attended influenza was 47% (95% CI 34%-57%), and 46% (95% CI 30%-58%) against the influenza A(H1N1)pdm09 strain. VE was also 44% (95% CI 13%-64%) against last year’s dominant strain, influenza A(H3N2), reported Joshua D. Doyle, MD, of the CDC, and colleagues, at multiple institutions nationwide.

For children ages 6 months to 17 years, overall adjusted vaccine effectiveness was 61% (95% CI 44%-73%), the authors wrote in the Morbidity and Mortality Weekly Report.

The authors said the overall VE estimate was “similar to the most recent A(H1N1)pdm09 predominant season (45%) in 2015–16.”

Doyle and colleagues performed an interim analysis of data from 3,254 children and adults in the U.S. Influenza Vaccine Effectiveness Network from November 23, 2018 to February 2, 2019. There were 465 who tested positive for influenza via real time reverse transcription polymerase chain reaction (RT-PCR) screening. Among 394 subtyped viruses, 74% were influenza A(H1N1)pdm09 viruses, and 26% were influenza A(H3N2) viruses.

Vaccine effectiveness against influenza A(H1N1)pdm09 was comparable for children (adjusted VE 62%, 95% CI 40%-75%). Among adults ages ≥50, however, VE against was much lower, to the point of insignificance, at 24% (95% CI -15% to 51%) for all subtypes and 8% for the A(H1N1)pdm09 strain (95% CI -59% to 46%).

Flu activity 2018-2019 so far

A second study in the MMWR by Lenee Blanton, MPH, of the CDC, and colleagues, provided an update on influenza activity, characterizing the 2018-2019 influenza season as “low severity” so far. Specifically, they noted relatively low rates of all key indicators — including deaths attributed to pneumonia and influenza, rates of hospitalization, and percentages of outpatient visits for influenza-like illness.

For the first time, the CDC is providing flu prevalence estimates, and as of February 2, the authors wrote, influenza is estimated to have caused 9,600-15,900 deaths and 155,000-186,000 hospitalizations.

Weekly percentages of pneumonia and influenza-associated deaths ranged from 5.5% to 7.4% from September 30, 2018 to January 26, 2019, though the authors noted that for 3 consecutive weeks in January, this was at or above the epidemic threshold.

There were 28 flu-related pediatric deaths reported to the CDC as of February 2, and among the 26 children with a known medical history, 46% had ≥1 underlying medical condition recognized by the Advisory Committee on Immunization Practices (ACIP). Vaccine history was available for 22 child deaths: six had received at least one dose of influenza vaccine prior to illness onset, the authors wrote.

There were 5,791 influenza-associated hospitalizations from October 2018 to February 2, 2019, for a cumulative rate of 20.1 per 100,000. Not surprisingly, the highest rate was among adults ages >65 (53.0 per 100,000). A large majority of the 755 hospitalized adults with information on underlying medical conditions had at least one underlying medical condition, with about 40% listed as having cardiovascular disease and 40% with obesity, the authors said.

This flu season, influenza A(H1N1) viruses have dominated in most areas of the country, with B viruses accounting for <3% of all influenza tests performed by public health laboratories, Blanton and colleagues wrote.