Patients should be screened regardless of age, gender, or time of year.
Although the frequency of some respiratory viruses remains consistent from year to year, others have clear demographic and seasonal patterns, and patients experiencing severe respiratory infections should be evaluated for a range of viruses, regardless of age, gender, or season. This is according to research results published in Influenza and Other Respiratory Viruses.
According to researchers, although the few long-term studies conducted on the epidemiology of respiratory viruses have demonstrated seasonality of circulation in defined geographic areas, most research on the topic covers shorter periods of time. Therefore, the aim of the current study was to determine the frequency and distribution of respiratory viruses in a Mediterranean climate over a 24-year period that included the first months of the COVID-19 pandemic.
Data were collected from January 1997 to March 2020 from a tertiary teaching hospital in Barcelona, Spain. Patients were grouped by age: 6 months and under, 6 to 12 months, 1 to 2 years, 3 to 5 years, 6 to 17 years, 18 to 29 years, 30 to 39 years, 40 to 59 years, and over 60 years of age. Clinical analyses of nasopharyngeal aspirates, nasal and pharyngeal exudates, bronchoalveolar lavages, and lung biopsy specimens were also conducted.
From 1997 to 2019, a total of 59,579 respiratory specimens were received (yearly median, 2590; range, 952-4883); 49,712 of these samples were conventionally processed and 9867 were processed using molecular methods. A total of 35.9% of samples were positive for at least one respiratory virus; 21,939 virus detections were made, and 2.6% of samples tested positive for the presence of more than 1 virus. Most samples were collected from adults aged 60 years and older; the fewest samples were collected from those in the 18 to 29 years age group. Frequency of detection for each virus varied by age group, with differences that were statistically significant.
In 2022, 66,616 specimens were collected—2903 from January to March—with 832 detections of respiratory viruses. All specimens that were received after the World Health Organization COVID-19 pandemic declaration were only analyzed for the presence of SARS-CoV-2, with a total of 6566 samples positive for SARS-CoV-2 in 2020.
The most frequently detected respiratory viruses were influenza A and respiratory syncytial virus (RSV; 31.4% and 28.7, respectively), co-detected in 25.5% of all samples positive for more than one virus. RSV, influenza A, adenoviruses, enteroviruses, and parainfluenza type 3 were detected every year throughout the period before the COVID-19 pandemic. RSV accounted for 15.5% to 47.1% of all detected respiratory viruses per year, with a significantly higher detection rate in 1997 and 2004. Influenza A diagnoses ranged from 4% to 56.4% with the highest rates in 1999, 2000, 2014, and 2019, and adenoviruses, enteroviruses, and parainfluenza virus type 3 ranged from 4.3% to 20.7%, 1.6% to 20%, and 1% to 7.2%, respectively. Following the introduction of routine testing for human metapneumovirus, rates of detection ranged from 2.7% to 7.9% between 2008 and 2019.
During the 23-year period prior to the COVID-19 pandemic, the relationship between season and prevalence was statistically significant for all viruses. Of the cold-weather viruses, RSV was most prevalent from November to January with a peak in December; influenza A was prevalent from December to March, with a peak in January, and influenza B was most prevalent from January to March, typically peaking 1 month after influenza A. Human metapneumovirus was most prevalent from February to May and parainfluenza virus type 2 was most prevalent from September to December. The other respiratory viruses that were detected—adenoviruses, enteroviruses, rhinoviruses, and parainfluenza virus types 1 and 2—were detected through the year, with peaks during various seasons.
To account for changes in circulation of viral illnesses during the COVID-19 pandemic, investigators separately analyzed and divided samples collected in 2020 into 2 periods: those from before the WHO COVID-19 pandemic declaration on March 15, 2020, and those from after. During the first period, 2903 specimens were analyzed for respiratory viruses that were not SARS-CoV-2; 28.6% of these were positive for viruses including influenza A and B, RSV, adenoviruses, human metapneumovirus, rhinoviruses, enteroviruses, and parainfluenza virus type 1. During the second period, 905 specimens were analyzed for other respiratory viruses, of which 3.3% were positive.
The investigators noted that the results for 2020 were not included in the statistical analysis.
Study limitations include that it was not based on systematic surveillance, as well as changes in methodology for influenza A and B and RSV during the study period that could have impacted the number of detections.
“This long-term study…broadens our knowledge of the prevalence of respiratory viruses in a Mediterranean region,” the researchers concluded. “Clear demographic and seasonal patterns were evident with some viruses…We conclude that patients [experiencing] severe respiratory infection should be screened for a wide range of respiratory viruses regardless of gender, age, and season.”