How does the flu vaccine affect the risk of contracting COVID-19?

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The coronavirus disease 2019 (COVID-19) is an acute respiratory disease that is caused by infection with the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). As a result of the rapid transmission of SARS-CoV-2 and its high morbidity and mortality rates, COVID-19 remains a significant threat.

Study: Does Influenza vaccination reduce the risk of contracting COVID-19? Image Credit: alessandro guerriero / Shutterstock.com

Background

Initially, the development of herd immunity once the vaccinated population reached 70% was believed to reduce the transmission of SARS-CoV-2.

However, the rate of vaccination in different nations varies depending on their specific economic situation. As a result, developed countries often have a higher vaccination rate as compared to underdeveloped countries. This increases the risk of prolonging the pandemic due to the emergence of mutated strains of SARS-CoV-2.

Several studies have observed that the risk of infection and disease transmission are independent of a COVID-19 vaccination regimen. Although the risk of developing a severe infection is lower in vaccinated people, the transmission of SARS-CoV-2 variants between vaccinated and unvaccinated individuals still occurs and can lead to the emergence of new variants.

Currently, ten SARS-CoV-2 variants are being monitored, two of which are considered variants of concern (VOCs). Thus, there remains an urgent need to better understand the evolution of SARS-CoV-2, as well as developing novel treatments and preventive measures that mitigate its spread and adverse effects on human health.

Previous studies have assessed whether countermeasures used to protect against other types of viruses could reduce the adverse effects of COVID-19. Influenza and COVID-19 are two respiratory viral diseases that have similar modes of transmission, clinical outcomes, hospitalizations, complications, and death rates. Additionally, the transmission of these viruses often peaks during the winter months.

The most effective strategy to prevent and control influenza epidemics is annual influenza vaccination. Similar vaccination campaigns have also been important in controlling the COVID-19 pandemic.

The SARS-CoV-2 Omicron variant, which was first detected in South Africa on November 24, 2021, acquired at least one of its mutations from the common cold virus that was also present in the same infected cell. The symptoms of Omicron infection are also very similar to that of the common cold.

Notably, flu vaccination has previously been found to reduce intensive care unit (ICU) admissions, hospitalizations, and mortality from SARS-CoV-2 infections. However, such findings are contradictory and insufficient. Further research is needed to better understand the protective role of the flu vaccine on COVID-19 infections.

A new Spanish study published on the preprint server medRxiv* describes the association between the risk of contracting COVID-19 and the seasonal flu vaccine in patients for whom the flu vaccine is recommended.

About the study

The current study involved patients who were considered high-risk for flu complications and were, as a result, recommended for vaccination. The clinical history of the patients was collected to obtain information on COVID-19 diagnosis and flu vaccination.

Study participants living in nursing homes were studied separately due to different living conditions. Both trivalent and quadrivalent flu vaccines were administered to the study participants.

The Mortality in small Spanish areas and Socioeconomic and Environmental Inequalities (MEDEA) deprivation index was used for the analysis of health inequalities and identification of regions with socioeconomic vulnerability. Information on the gender, age, risk factors for flu vaccination, the incidence of COVID-19, and flu vaccine receipt were also collected.

Study findings

Out of the 429,537 study participants, about 45% were vaccinated against the flu, while 56.8% were women. Participants over the age of 80 were most likely to be vaccinated. Moreover, about 4% of vaccinated people and 4.44% of unvaccinated people contracted COVID-19.

Patients with risk factors for flu complications, such as those with cardiovascular diseases, lung problems, diabetes mellitus, and kidney problems, who also received the flu vaccine were at a lower risk of contracting COVID-19.

Probability of contracting COVID-19 depending on whether or not the patient had received the flu vaccine. Probability of contracting COVID-19 depending on whether or not the patient had received the flu vaccine.

High MEDEA values ​​were associated with lower vaccination rates and a higher risk of COVID-19. Additionally, at high MEDEA index values, people under 60 years of age were associated with higher COVID-19 incidence rates as compared to those over the age of 60.

Conclusions

The current study demonstrates that the flu vaccine could reduce the risk of COVID-19 among individuals who are at high risk for flu complications. However, continuous and effective immunizations are required to reduce the burden of respiratory diseases, especially during a period in which influenza and COVID-19 overlap.

Further research is needed to develop preventive strategies against both diseases simultaneously, as there is a high probability of living with both viruses for a long time.

*Important notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

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