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As influenza A(H3N2) remains the most prevalent subtype in the US, investigators estimated the available flu shot is not significantly preventing positive cases.
Interim estimate data for the 2021-22 US influenza season suggest the available flu vaccine did not provide significant efficacy in preventing outpatient medically-attended illnesses.
The new Centers for Disease Control (CDC) findings point to the burden of evasiveness infection and illness wrought by the dominant influenza A(H3N2) virus strain this season—though the investigators stressed vaccination is still a worthwhile measure for unprotected persons at this point in the season.
Led by Jessie R. Chung, MPH, of the Influenza Division at the CDC National Center for Immunization and Respiratory Diseases, investigators sought to estimate this flu season’s vaccine efficacy against laboratory-confirmed, mild-to-moderate acute respiratory infection (ARI)—a practice conducted annually by the CDC since 2004-05.
Such estimates were not accumulated for the 2020-21 flu season due to insufficient data during the COVID-19 pandemic.
The team used interim data of 3636 children and adults with diagnosed ARI enrolled in the Influenza Vaccine Effectiveness Network from October 4, 2021 to February 12, 2022 to interpret vaccine efficacy. Patient data was collected from 1 of 7 study sites across the US.
Investigators observed 194 (5%) positive test results for influenza A virus infection via real-time reverse-transcription polymerase chain reaction (RT-PCR). None tested positive for influenza B virus infection. Another 11 patients tested positive both influenza A and SARS-CoV-2 viruses. Of the 178 influenza A viruses subtyped, all but 1 were identified as A(H3N2).
Just 41% of participants to test positive for influenza had received a flu shot this season, versus 50% of participants who tested negative. Vaccine efficacy against outpatient medically-attended ARI associated with influenza A virus was just 14% (95% CI, -17 to 37). Against subtype A(H3N2), vaccine efficacy was 16% (95% CI, -16 to 39).
As has been previously observed in the 2021-22 flu season, influenza A(H3N2) is the most prevalent strain in the US right now; though cases remain relatively low compared to past flu seasons, experts have explained to HCPLive® that A(H3N2) is a particularly troubling virus subtype.
Tim Uyeki, MD, MPH, MPP, Chief Medical Officer of the CDC’s Influenza Division, National Center for Immunization and Respiratory Diseases, said last year that the most severe flu epidemics in recorded history were linked to this subtype.
“The bottom line is we could be at risk on a population basis of a rather severe influenza epidemic with type A (H3N2) viruses, depending upon how antigenically similar the viruses are compared to what people have been exposed to in the past, and how well the H2N3 vaccine strain has been mapped to it,” Uyeki explained at the time.
In these newest CDC findings, Chung and colleagues noted that flu vaccination coverage remains lower this season among high-risk groups including pregnant women, infants and pre-school children, and patients from racial and ethnic minority groups.
“CDC continues to recommend influenza vaccination when (vaccine efficacy) against outpatient illness is reduced because a growing body of evidence suggests that influenza vaccination can avert serious outcomes, including hospitalization, ICU admission, and death, among persons who are vaccinated but still become infected,” they wrote. “In addition, vaccination is likely to prevent illness or serious complications of infection with other influenza viruses that might circulate later in the season, including influenza A(H1N1)pdm09 and B viruses.”
The team identified 4 study limitations, including a limitation of vaccine efficacy estimations to only influenza A and subtype A(H3N2); the self-reporting nature of recording flu shots; patients’ health care-seeking behavior altered during the COVID-19 pandemic affecting outpatient care rates; and the limitation of influenza illnesses assessed to interpret vaccine effectiveness.
That said, the findings do indicate that clinicians should not limit their awareness of nor response to flu cases based on a patients’ vaccination status this season.
“Physicians should not wait for confirmatory influenza laboratory testing, and the decision to use antiviral medication should not be influenced by patient influenza vaccination status,” they concluded. “Clinicians should be aware that influenza activity might continue or increase, and influenza should be considered as a possible diagnosis in all patients with ARI.”
The study, “Interim Estimates of 2021–22 Seasonal Influenza Vaccine Effectiveness — United States, February 2022,” was published online in Morbidity and Mortality Weekly Report.