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Influenza vaccine effectiveness in the course of the 2021–2022 influenza season in america

In a current article revealed in Medical Infectious Ailments, researchers estimated influenza vaccine effectiveness (VE) in sufferers aged ≥6 months searching for outpatient look after acute respiratory sickness (ARI) because of the influenza A(H3N2) virus between October 2021 and April 2022 in america (US).

Examine: Influenza Vaccine Effectiveness Towards Influenza A(H3N2)-Associated Sickness in america In the course of the 2021–2022 Influenza Season. Picture Credit score: Andrey_Popov/Shutterstock


Because the starting of the extreme acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic, the US has skilled meek influenza exercise. Nonetheless, it reemerged throughout 2021-2022 in the course of the predominance of Omicron, leading to an estimated eight to 13 million influenza circumstances and three.7 to six.1 million influenza-related hospital visits. Thus, this time was favorable for estimating VE towards influenza A(H3N2) regardless of the sturdy correlation between vaccines of those two viruses.

In regards to the examine

Within the current examine, the researcher employees screened all sufferers over six months previous who sought outpatient medical care or scientific SARS-CoV-2 testing at a US Flu VE Community hospital throughout seven US states. They performed the examine amongst all sufferers aged ≥6 months who had ARI, cough, fever, and ageusia or anosmia for VE estimates for the 2021–2022 influenza season. Moreover, they stratified these estimates by age and early vs. late influenza exercise.

The crew collected oropharyngeal swabs however solely nasal swabs from kids beneath two years. They used them to check for influenza and coronavirus illness 2019 (COVID-19) by way of reverse-transcriptase polymerase chain response (RT-PCR). They included those that had cough or fever, sought scientific testing inside seven days of sickness onset, and excluded those that have been SARS-CoV-2-positive from the VE evaluation. The management group comprised sufferers testing destructive for influenza and SARS-CoV-2.

The researchers used logistic regression fashions incorporating a test-negative design to estimate influenza vaccine VE by 100% x (1 – adjusted odds ratio [OR]). Moreover, they carried out a sensitivity evaluation with individuals constructive for SARS-CoV-2.

Examine findings

Although its circulation remained low relative to SARS-CoV-2, influenza A(H3N2) viruses have been predominant in the course of the 2021-2022 period. Accordingly, influenza positivity ranged between 0% and fewer than 10% weekly all through the examine interval peaking in mid-January 2022.

Though 7031 individuals enrolled within the examine, 6244 individuals made it to the ultimate dataset. Of those, 7% (468) examined constructive for influenza, and solely 440 have been A(H3N2) circumstances. Complete genome sequencing characterised that 47% of the influenza A(H3N2) circumstances belonged to 3C.2a1b subclade 2a.2.

Although the present examine dataset was not giant sufficient for detecting statistically important VE under 30% even amid excessive vaccine protection, the researchers made two key observations. First, the influenza vaccines displayed a VE of 36% towards A(H3N2)-induced sicknesses in all individuals beneath 50 years. Second, amongst adults ≥ 50 years, the noticed safety towards RT-PCR-confirmed influenza was statistically insignificant.

Most 2021-2022 egg-, cell- and recombinant-based influenza vaccines used the identical clade, and its uptake was greater amongst US Flu VE Community individuals, together with older adults. Whereas the vaccine element used clade 3C.2a1b subclade 2a.1, H3N2 viruses circulating within the US throughout this time have been of subclade 2a.2. It remained unclear whether or not adults ≥ 50 years responded aberrantly to the mismatched vaccine element or elicited completely different cross-protective antibodies.

Curiously, the elimination of individuals constructive for SARS-CoV-2 elevated influenza VE level estimates by >5% between October 2021 and April 2022. These individuals have been much less prone to obtain influenza vaccine leading to decrease estimates of influenza VE, a bias that had the best impact throughout excessive COVID-19 prevalence.


Total, influenza vaccines within the 2021–2022 season conferred related safety because the vaccines within the influenza A(H3N2) seasons earlier than the COVID-19 pandemic started. It lower down ARIs with a cough by one-third total. Within the 2022-2023 influenza season, updating the A(H3N2) vaccines with a element representing the 2a.2 subclade would possibly enhance the conferred safety, particularly if the same clade of viruses continues to flow into within the US.



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