Researchers Investigate the Striking Absence of Flu and Other “Usual Suspects” During COVID-19 Pandemic

By European Society of Clinical Microbiology and Infectious Diseases July 7, 2021

Influenza and respiratory syncytial virus (RSV) cases dropped significantly in both adults and children during the COVID-19 pandemic, according to new research presented online this year at the European Congress of Clinical Microbiology & Infectious Diseases (ECCMID).

Influenza A and influenza B (both of which cause the flu) and RSV are common causes of respiratory infections, especially in winter. There are 9 to 45 million cases of the flu in the US each year. RSV is especially common in children under two, but it can affect people of all ages.

The retrospective study, by Ms. Siri Sarvepalli and colleagues from Wayne State University, Detroit, USA, looked at whether the incidence of these and other respiratory infections decreased during the COVID-19 pandemic.

It involved comparing the results of PCR tests for various infections conducted between September 2019 and February 2020 at the Detroit Medical Center (DMC) and the Children’s Hospital of Michigan (CHM) with the results from September 2020 to February 2021.

The PCR tests used nasopharyngeal swab samples to test for influenza A and B, RSV, and SARS-COV-2. Throat swab samples were used in PCR testing for Group A Streptococcus (GAS), a bacterial throat infection that typically follows viral upper respiratory tract infections.

Data on other respiratory virus infections were also reviewed.

For adults, in the 2019-2020 season, 11.5% of influenza A, 13.1% of influenza B, and 9% of RSV tests were positive. In the 2020-2021 season, 0% of influenza A, influenza B and RSV tests were positive.

For children, in the 2019-2020 season, 12.4% of influenza A, 20.2% of influenza B, and 23.7% of RSV tests were positive. In the 2020-2021 season, 0% of influenza A and influenza B returned positive. One RSV test was positive.

The number of tests for GAS has been drastically reduced from the previous year. In the period 2019-20, some 933 GAS tests were performed in adults (of which 22.8% positive), compared to 212 (23.11% positive) a year later. For children, the number dropped from 3,984 tests (27% positive) to 777 (20.98% positive). See table in the link below.

The incidence of other, less common, respiratory viruses had also fallen. In 2019-2020, the proportion of tests for other respiratory viruses that came back positive ranged from 0.2-4.2% (Parainfluenza virus 1: 3.5%, parainfluenza virus 2: 0.4%, human metapneumovirus: 4. 2%, Coronavirus 229E: 0.2%) compared to 0% in 2020-2021 – a change described as “remarkable” by the study authors. Data on these infections from 42 medical centers in the US Midwest revealed a similar pattern across the region. (See table 3 of the poster above.)

The researchers say: “The incidence of influenza A and B and RSV infections in the 2020-2021 season decreased significantly compared to the 2019-2020 season in both pediatric and adult populations. Community restrictive measures, such as social distance, school closures and the use of masks may have reduced the spread of viral pathogens, which may also explain the decrease in GAS infections, which usually occur after infection with a respiratory virus.”

In addition, infection with COVID-19 may have protected people from other respiratory viruses, through viral interference. This happens when a virus temporarily blocks a co-infection of another virus and was seen during the H1N1 “swine flu” pandemic in 2009.

Similar reductions in the incidence of flu and other respiratory viral infections have been seen elsewhere, including in the Southern Hemisphere, where Australia, Chile and South Africa reported a total of just 51 cases of flu in the 2020 flu season.

Ms Sarvepalli said: “It is likely that the incidence of flu and other respiratory infections will return to normal in the coming years as SARS-CoV-2 becomes a seasonal virus.

“However, if handwashing and other mitigation measures are followed to the same extent as last winter, numbers could remain lower than normal.”

This article is based on poster presentation 2678 at the European Congress of Clinical Microbiology & Infectious Diseases (ECCMID). The material has been peer-reviewed by the congress selection committee. The study has been submitted to a journal, but the full article is not available at this stage.