Influenza vaccine effectiveness throughout the 2021–2022 influenza season in the USA

In a current article revealed in Clinical Infectious Diseases, researchers estimated influenza vaccine effectiveness (VE) in sufferers aged ≥6 months looking for outpatient take care of acute respiratory sickness (ARI) because of the influenza A(H3N2) virus between October 2021 and April 2022 in the USA (US).

Study: Influenza Vaccine Effectiveness Against Influenza A(H3N2)-Related Illness in the United States During the 2021–2022 Influenza Season. Image Credit: Andrey_Popov/Shutterstock
Research: Influenza Vaccine Effectiveness Against Influenza A(H3N2)-Related Illness in the United States During 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. Nevertheless, it reemerged throughout 2021-2022 throughout 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 in opposition to influenza A(H3N2) regardless of the sturdy correlation between vaccines of those two viruses.

In regards to the research

Within the current research, the researcher workers screened all sufferers over six months outdated who sought outpatient medical care or scientific SARS-CoV-2 testing at a US Flu VE Community hospital throughout seven US states.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 research 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 youngsters underneath two years. They used them to check for influenza and coronavirus illness 2019 (COVID-19) through 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 had been SARS-CoV-2-positive from the VE evaluation. The management group comprised sufferers testing detrimental 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 optimistic for SARS-CoV-2.

Research findings

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

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

Although the present research 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% in opposition to A(H3N2)-induced diseases in all individuals underneath 50 years. Second, amongst adults ≥ 50 years, the noticed safety in opposition to 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 increased amongst US Flu VE Community individuals, together with older adults. Whereas the vaccine part used clade 3C.2a1b subclade 2a.1, H3N2 viruses circulating within the US throughout this time had been of subclade 2a.2. It remained unclear whether or not adults ≥ 50 years responded aberrantly to the mismatched vaccine part or elicited completely different cross-protective antibodies.

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


General, 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 minimize down ARIs with a cough by one-third general. Within the 2022-2023 influenza season, updating the A(H3N2) vaccines with a part 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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