Impact of the COVID-19 Pandemic on Respiratory Infection Rates.

To the Editor:

A substantial change in the pattern of influenza infections was noted worldwide during the COVID-19 pandemic. We sought to describe our experience with seasonal respiratory infections across 60 sites of a major health care system in the Midwest United States. Although timing varies slightly from year to year, seasonal acute respiratory infections, such as influenza A/B, respiratory syncytial virus (RSV), and group A streptococcus (GAS), have predictable months of increased activity. We collected the number of tests performed for influenza A/B, RSV, GAS, and SARS-CoV-2 from November 1, 2018, to March 31, 2020 (ie, the pre-pandemic period) and April 1, 2020, to June 24, 2021 (ie, the pandemic period). During the COVID-19 pandemic, we observed lower rates of influenza A/B than previously reported and discovered substantially decreased positivity rates for RSV and GAS as well.1, 2, 3, 4

A total of 1,489,293 tests were performed for influenza A/B, RSV, GAS, and SARS-CoV-2 between November 1, 2018, and June 24, 2021 (Table ; Supplemental Figure, available online at http://www.mayoclinicproceedings.org). Our pre-pandemic interval includes 2 typical respiratory seasons with peaks from November through March, during which time 11% of all influenza tests returned positive results, 8% of RSV test results were positive, and 25% of GAS test results were positive. In comparison, during the COVID-19 pandemic, there were only 28 (0.02%) positive influenza results of 139,816 tests, 24 (0.2%) positive RSV results of 11,412 tests, and 5622 (13.2%) positive cases of GAS of 42,485 tests. Even though the overall rate of testing of influenza A/B and RSV was lower during the pandemic, the positivity rate decreased significantly (P<.001), more than would be predicted, and approached zero.

Table

Tests for Acute Respiratory Infections Performed and Results

	Pre-pandemic perioda (n=350,334)	Pandemic periodb,c (n=1,138,959)	Overall (N=1,489,293)
Test performed
Influenza A/B	20,8438 (59.5)	139,816 (12.3)	348,254 (23.4)
Respiratory syncytial virus	37,717 (10.8)	11,436 (1.0)	49,153 (3.3)
Group A streptococcus	93,392 (26.7)	42,485 (3.7)	135,877 (9.1)
SARS-CoV-2	10,787 (3.1)	945,222 (83.0)	956,009 (64.2)
Influenza A/B test results
Negative	185,311 (88.9)	139,788 (100.0)	325,099 (93.4)
Positive	23,127 (11.1)	28 (0.0)	23,155 (6.6)
Respiratory syncytial virus test results
Negative	34,711 (92.0)	11,412 (99.8)	46,123 (93.8)
Positive	3006 (8.0)	24 (0.2)	3030 (6.2)
Group A streptococcus test results
Negative	70,306 (75.3)	36,863 (86.8)	107,169 (78.9)
Positive	23,086 (24.7)	5622 (13.2)	28,708 (21.1)
SARS-CoV-2 test results
Negative	10,739 (99.6)	865,086 (91.5)	875,825 (91.6)
Positive	48 (0.4)	80,136 (8.5)	80,184 (8.4)

Values are reported as number (percentage).

Pre-pandemic period: November 1, 2018, to March 31, 2020.

Pandemic period: April 1, 2020, to June 24, 2021.

All P values are <.001.

Patients tested for non-COVID viruses during the pandemic tended to be older (median age, 41.7 years; Q1-Q3, 19.9-66.5 years) compared with patients tested during the pre-pandemic period (median age, 28.7 years; Q1-Q3, 7.9-59.4 years; Wilcoxon, P<.001). However, there was no age difference in comparing positive non-COVID test results between the pandemic (median age, 14.6 years; Q1-Q3, 8.3-25.9 years) and the pre-pandemic periods (median age, 12.8 years; Q1-Q3, 6.2-35.5 years, Wilcoxon, P=.466).

We also investigated the possibility of viral interference by SARS-CoV-2 on the rate of respiratory viral infections. To assess this, we compared positivity rates of influenza A/B and RSV between patients who had previously tested positive for SARS-CoV-2 and patients who had not tested positive. Only respiratory tests conducted during the pandemic period were included in this subanalysis. To serve as a baseline, we also compared positivity rates among all patients during the pre-pandemic period with patients in the pandemic period who had no previous positive SARS-CoV-2 test results. We found that viral interference did not contribute in decreasing the rates of influenza or RSV.

In conclusion, there was a significant decrease observed in the rate of positivity of 3 respiratory illnesses before and during the COVID-19 pandemic. The explanation for the decreased rates of influenza A/B, RSV, and GAS is likely to be multifactorial, and further work will be needed to determine all the elements influencing this change as we navigate future respiratory seasons.