COVID-19 Mitigation Efforts Significantly Reduce Flu Activity

Mitigation strategies in place to slow the spread of COVID-19 appear to reduce influenza transmission in the United States, Australia, Chile, and South Africa. These strategies can increase flu vaccines, especially for high-risk populations during seasonal flu transmission.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), a new coronavirus, was first detected in Wuhan, Hubei province, People’s Republic of China, in late December 2019. Later, SARS-CoV- 2 spread rapidly, prompting COVID. -19 pandemic. COVID-19 is overshadowing other similar respiratory illnesses like influenza. Sonja J. Olsen, et al. report in the CDC Morbidity and Mortality Weekly Report that positive influenza cases (from US respiratory samples) have dropped from> 20% to 2.3% during this pandemic.

Southern hemisphere countries also report the same observation, with little influenza activity. A low incidence of flu symptoms is reported, and in those tested for flu, few were positive. Interventions targeting SARS-CoV-2 transmission, plus influenza vaccination, could be the reason for low influenza activity in the United States and around the world.

Mitigations against the COVID-19 pandemic included individual measures such as the use of masks, staying home during illness and social distancing, and community measures such as school closings, mass gathering bans and stay-at-home orders, and others. such as mandatory 14-day travel quarantines, national curfews and closures.

Initially, people with respiratory symptoms were preferably tested for SARS-CoV-2. This could be attributed to the observed decrease in the influenza virus. However, later, public health officials and doctors began testing samples for influenza. Adequate numbers were tested and little or no influenza viruses were detected compared to the incidence of influenza activity the previous year.

Australia tested more samples for influenza, usually when the southern hemisphere influenza epidemics peak. They also detected only a few positive results, despite stringent low criteria for testing respiratory samples compared to previous seasons.

Although the temperate climates of the southern hemisphere have practically no circulation of the influenza virus, it is observed that it decreases in the United States, other countries of the northern hemisphere, and in the tropics.

The decline in influenza may be due to a reduction in people seeking or reporting respiratory illness, as well as changes in the circulation of the influenza virus. Influenza virus (Ro = 1.28) is low compared to SARS-CoV-2 (R₀ = 2–3.5). Thus, although influenza is also transmitted primarily by droplets, like SARS-CoV-2, mitigation measures taken during this pandemic period have substantially reduced influenza transmission.

Number of respiratory samples tested and percentage of positive tests for influenza, by year – United States, seasons 2016-17 to 2019-2020

Importance of the report

Populations at high risk of developing serious illness and complications from influenza can continue to adapt mitigation measures during the influenza season in addition to receiving the influenza vaccine. This can be helpful for the elderly and at high risk, as well as reducing the burden of healthcare support.

Number of samples tested and percentage of positive tests for influenza, by year: Australia, Chile and South Africa, April-August (weeks 14 to 31), 2017-20

The authors agree that it is difficult to separate the effect of general mitigation measures on influenza transmission during this season. For example, school-age children spread influenza and closing schools can effectively reduce the spread of influenza. However, because adults have other exposures, the effectiveness of influenza transmission this season cannot be demonstrated.

According to the authors, the findings in this report have at least four limitations:

1. Although the findings in several countries are consistent and convincing, an ecological analysis cannot determine causality;
2. A sharp decline in global travel and increased use of flu vaccines, factors not taken into account in this study, may play an important role in reducing flu
3. Virus interference: This could help explain the lack of influenza during a pandemic caused by another respiratory virus that could overtake influenza in the respiratory tract.
4. It is also possible that the decline in influenza seen in the United States is just the natural end of the flu season. However, after March 1, 2020 (declaration of a national emergency related to COVID-19), a sharp decrease in positive influenza cases was observed.

The spread of influenza is still being monitored to determine if low-level activity persists even after mitigation measures are removed. However, in the current uncertain circumstances, it is vital to plan and prepare for the circulation of seasonal influenza, especially in the 2020-21 Northern Hemisphere season. Additionally, a new Food and Drug Administration (FDA) -approved multiplex diagnostic assay for the detection of SARS-CoV-2 and influenza viruses could improve predictive efforts and also direct our mitigation strategies for both diseases.

With the looming fact that SARS-CoV-2 and the influenza virus could co-circulate this season, the best method for preventing influenza is influenza vaccination (for all persons ≥6 months). Taking the limitations into account, the authors conclude that the widespread implementation of measures to mitigate the transmission of SARS-CoV-2 is associated with the decrease in the incidence of influenza, which appears to be real and concurrent with the COVID-19 pandemic.