The Effect of Influenza Vaccination on COVID-19 Morbidity, Severity and Mortality: Systematic Review and Meta-Analysis

Influenza vaccination and COVID-19 infection risk and disease severity: A systematic review and multilevel meta-analysis of prospective studies

BACKGROUND

In light of the Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) pandemic, the influence of influenza vaccination on the risk and severity of Coronavirus Disease 19 (COVID-19) has been a subject of debate. This systematic review and meta-analysis of prospective studies aim to assess the association between influenza immunization and the risk of SARS-CoV-2 infection and subsequent COVID-19 disease severity.

METHODS

A comprehensive search of PubMed and Embase databases was performed to identify prospective studies published before March 2024. We focused on evaluating the effect of influenza vaccination on SARS-CoV-2 infection risk and severe COVID-19 outcomes, such as hospitalization and mortality. The analysis employed a multilevel random effects meta-analysis approach. The risk of bias assessment was conducted using the Newcastle-Ottawa Scale.

RESULTS

From an initial pool of 5,863 records, 14 studies were selected for inclusion. The aggregated data yielded a summary relative risk (SRR) that showed no significant protective correlation between influenza vaccination and SARS-CoV-2 infection risk (SRR 0.95, 95% confidence interval [CI] 0.81-1.11), COVID-19-associated hospitalization (SRR 0.90, 95% CI 0.68-1.19), or COVID-19-related mortality (SRR 0.83, 95% CI 0.56-1.23).

CONCLUSIONS

This systematic review and meta-analysis, based exclusively on prospective studies, demonstrates the lack of a proven protective effect of influenza vaccination against COVID-19 and related outcomes. Our results do not support a significant protective effect of influenza vaccination against the risk or severe outcomes of COVID-19.

Flu vaccine administration in the period before SARS-CoV-2 infection and its outcomes: An umbrella review

Objective

The potential association between influenza vaccination and SARS-CoV-2 infection and related outcomes is still controversial. The aim of this umbrella review is to represent the impact of previous influenza vaccination and COVID-19 outcomes using evidence currently available in literature.

Methods

A literature search of MEDLINE, EMBASE, Scopus, Web of Science and Cochrane Library was conducted. The paper selection was conducted using the preferred reporting items for systematic reviews and meta-analyses (PRISMA) method by two-blinded authors. The quality of meta-analyses was assessed using the AMSTAR 2 scale (A MeaSurement Tool to Assess systematic Reviews). The outcomes investigated were SARS-CoV-2 infection after influenza vaccination, hospitalization, intensive care unit admission, mechanical ventilation and mortality.

Results

The literature research identified 7 ecological studies and 6 meta-analyses. All the ecological studies show a negative relationship between influenza vaccination and COVID-19. The meta-analyses suggest a protective action of influenza vaccination against SARS-CoV-2 infection. Regarding the outcomes evaluated, only two studies reported a statistically significant reduction of 12% and of 17% in hospitalization and intensive care unit admission, respectively. Regarding mechanical ventilation, three studies showed a risk reduction of 31%, 27% and 28%. A substantial reduction of mortality risk was also observed in one study.

Conclusions

These results suggest that influenza vaccination could be associated with reduced susceptibility to SARS-CoV-2 infection, mechanical ventilation and mortality. Our findings highlighted how the administration of flu vaccine in subjects at risk could lead to a reduction in mortality, particularly in the over 65y.

A Cross-Sectional Survey of Knowledge, Attitude, and Practices Regarding Influenza Vaccination Among Jordanians Aged 18-64 Years with Chronic Diseases

Objectives

Influenza is a frequent infectious disease that can be prevented and is associated with significant mortality and morbidity. The most economical way to prevent influenza is through vaccination, although this method is not widely used. This study aimed to assess the seasonal influenza vaccination rates and the knowledge and attitudes of Jordanian adults with chronic illnesses toward the influenza vaccine.

Materials and Methods

A cross-sectional design was employed. A 26-item online survey was used to gather data about patients' knowledge of and attitudes toward the influenza vaccine as well as their status as influenza vaccine recipients.

Results

A total of 19% of the 564 study participants received an influenza vaccination. Most (81%) of individuals reported inconsistent vaccination uptake. The most important factor affecting vaccination is the belief that the flu is not a threat (39%) and that their doctors did not advise them about the vaccination (32%). Participants with no health insurance and those with public insurance had a lower level of vaccination than those with private insurance (p= 0.008).

Conclusion

The adult population of Jordan with chronic diseases has subpar immunization rates. What is also revealed is a blatant misunderstanding about the value of routine influenza vaccination. These findings emphasize how urgently the public needs to be made aware of the effectiveness of the influenza vaccine.

Relationship between Humoral Response in COVID-19 and Seasonal Influenza Vaccination

There is evidence that vaccination against seasonal influenza can improve innate immune responses to COVID-19 and decrease disease severity. However, less is known about whether it could also impact the humoral immunity in SARS-CoV-2 infected patients. The present study aimed to compare the SARS-CoV-2 specific humoral responses (IgG antibodies against nucleocapsid; anti-N, receptor binding domain; anti-RBD, subunit S2; anti-S2, and envelope protein; anti-E) between non-hospitalized, COVID-19 unvaccinated, and mild COVID-19 convalescent patients who were and were not vaccinated against influenza during the 2019/2020 epidemic season (n = 489 and n = 292, respectively). The influenza-vaccinated group had significantly higher frequency and titers of anti-N antibodies (75 vs. 66%; mean 559 vs. 520 U/mL) and anti-RBD antibodies (85 vs. 76%; mean 580 vs. 540 U/mL). The prevalence and concentrations of anti-S2 and anti-E antibodies did not differ between groups (40-43%; mean 370-375 U/mL and 1.4-1.7%; mean 261-294 U/mL) and were significantly lower compared to those of anti-RBD and anti-N. In both groups, age, comorbidities, and gender did not affect the prevalence and concentrations of studied antibodies. The results indicate that influenza vaccination can improve serum antibody levels produced in response to SARS-CoV-2 infection.