Chinese Medical Journal study explores the association of influenza vaccination with COVID-19 infection and associated outcomes

The COVID-19 pandemic, caused by the severe acute respiratory syndrome coronavirus (SARS-CoV-2), has infected billions of people across the world, placing a tremendous burden on global health systems. However, non-pharmaceutical interventions (NPI) like masking, washing one's hands, and social distancing have played a key role in containing the transmission of this virus.

Once these public health measures are relaxed, populations globally could be at a greater risk of contracting other respiratory diseases with the same mode of transmission, such as influenza.

In particular, getting co-infected with SARS-CoV-2 and the influenza virus could worsen the ongoing symptoms to a large extent, making treatment difficult. To put this into perspective, co-infection with the influenza virus increases the likelihood of needing invasive mechanical ventilation by 4 times, and the risk of mortality by 2.35 times, compared to SARS-CoV-2 infection alone.

Interestingly, both SARS-CoV-2 and influenza virus share certain similarities. For starters, both viruses use surface glycoproteins to bind to cell membrane receptors of the respiratory system while initiating the infection. In fact, certain anti-influenza drugs are used clinically to treat COVID-19 patients, pointing to the possibility of a common intervention against both these viruses. Moreover, studies have shown that higher vaccination rates against influenza are associated with a decrease in cumulative incidence, morbidity, or death due to COVID-19.

These observations point to a very important question: Can influenza vaccines reduce the likelihood of COVID-19 infection and the severity of outcomes linked to COVID-19? 

Through a new study published in Chinese Medical Journal on November 16, 2022, a team of researchers from the Chinese Academy of Medical Sciences and Peking Union Medical College aimed to answer this question. The team, which was led by Professor Luzhao Feng, conducted a systematic review to determine if vaccination against the influenza virus could reduce the initial SARS-CoV-2 infection and its associated outcomes in the following year.  For this purpose, they sourced related studies published up to May 2022, from four electronic databases, including the Cochrane Library, PubMed, Embase, and Web of Science

The team selected a total of 36 studies with data on 55,996,841 individuals, none of whom were vaccinated against COVID-19, and conducted a meta-analysis to pool all the odds ratios (ORs) and confidence intervals.

They found that being vaccinated against the influenza virus in 2019 or 2020 reduced the risk of SARS-CoV-2 infection by 20% within a year, corresponding to an OR of 0.80. In addition, influenza vaccination was found to be negatively associated with poor outcomes. For patients with COVID-19 who were vaccinated against the influenza virus, the risk of intensive care unit admission was reduced by 17%, and that of needing ventilator support as well as the likelihood of death was reduced by 31%, corresponding to ORs of 0.83, 0.69, and 0.69 respectively. The findings pertaining to the severity of outcomes were inconsistent, primarily due to a limited number of studies and their heterogenous nature.

“Although influenza vaccines provide specific protection against the influenza virus, they also extend a non-specific immunity which acts against certain symptoms of COVID-19. Hence, healthcare centres should consider providing routine influenza vaccination along with that for COVID-19 as a precautionary measure,” remarked Binshan Jiang while discussing these findings.

To sum up, this meta-analysis shows promise that the likelihood of severe outcomes and fatality associated with COVID-19 are lower among patients vaccinated against the influenza virus.

What are the future implications of this study? The authors added, “The findings from this study could be used for the development of a broad vaccine which offers protection against multiple respiratory diseases including COVID-19. Until then, additional studies are required to understand the differences and interactions of these two vaccines, to improve the protection offered by them.”

Although a lot is yet to be uncovered regarding the mechanism through which influenza vaccines reduce the severity of the COVID-19 infection, it is clear that an annual influenza vaccination regimen can offer various benefits in these turbulent times.

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Reference

DOI: https://doi.org/10.1097/CM9.0000000000002427