The effect of influenza vaccine on severity of COVID-19 infection: An original study from Iran

The COVID-19 infection is a novel virus that mainly targets the respiratory system via specific receptors without any coronavirus-targeted therapies. Many efforts have been made to prepare specific vaccines for COVID-19 or use of prefabricated vaccines of other similar viruses, especially severe acute respiratory syndrome (SARS), Middle East respiratory syndrome (MERS), and influenza (flu). We aimed to evaluate the effects of previous flu vaccine injection on severity of incoming COVID-19 infection.

We conducted a large cross-sectional study of 529 hospitalized Iranian COVID patients to evaluate the severity of disease courses in patients with or without previous flu vaccination history using some main factors like length of hospitalization, need for the intensive care unit (ICU) admission and length of stay in the ICU for comparison between COVID-19 infected patients with or without flu vaccination history. For the quantitative data, we used independent-samples t and Mann-Whitney tests. The qualitative data were calculated using the Fisher exact and chi-square tests in IBM SPSS Statistics version 22 (SPSS Inc) and P value <0.05 was considered statistically significant.

There were no significant differences in the demographic data of patients, disease, and severity-related parameters between the 2 groups. It means that there were not any significant differences between patients with and without history of flu vaccination regarding mean days of hospitalization, percentage of needing to be admitted to the ICU, days being admitted to the ICU (8.44±6.36 vs 7.94±8.57; 17% vs 11.5%; and 1.17±3.09 vs 0.92±3.04, retrospectively) (p=0.883, 0.235, and 0.809, respectively).
In the laboratory tests, in comparison between patients with and without history of previous flu vaccination, only lymphocytes count in the vaccine positive group was higher than the vaccine negative group (20.82±11.23 vs 18.04±9.71) (p=0.067) and creatine phosphokinase (CPK) levels were higher in the vaccine negative group (146.57±109.72 vs 214.15±332.06) (p=0.006).

We did not find any association between flu vaccination and decrease in disease severity in our patients.  It seems that patients with previous history of flu vaccination may experience less laboratory abnormalities in some parameters that could be interpreted in favor of lower overall inflammation; however, this study cannot answer this definitely because of its design. As we collected retrospective data from only alive discharged patients and had no healthy control group, we could not discuss the probable effect of the vaccine on the mortality rate or its probable protective role against the infection. We need more well-designed controlled studies with different populations in different geographic areas to address the controversies.