Background
Coronaviruses (CoVs) circulate mainly in birds and mammals but can sometimes evolve, cross the barrier between species, and infect humans1. Until a few months ago, six different species of CoV were known to affect humans (human (H)CoV-229E, HCoV-NL63, HCoV-OC43, HCoV-HKU1, severe acute respiratory syndrome (SARS)-CoV, and Middle East Respiratory Syndrome CoV)2. However, in December 2019, cases of pneumonia without a known cause began to appear, epidemiologically linked to a fish and animal market in Wuhan, Hubei Province, China. Later, through the genetic sequencing of the isolates obtained from the patients, the aetiological agent was identified as a novel CoV species genetically close to CoVs of bats, which was named SARS-CoV-2, and the disease it causes was named coronavirus disease 2019 (COVID-19)3-5.
On March 11, 2020, the WHO declared the outbreak of COVID-19 a pandemic, and by April 6, 2020, more than 1 million confirmed cases had been reported. The main clinical symptoms of the confirmed cases are fever, cough, shortness of breath, fatigue, and gastrointestinal symptoms6.
This new disease can vary from asymptomatic to severe respiratory disease with multiorgan failure, and advanced age and some comorbidities, such as diabetes and hypertension, seem to be associated with more severe cases and death6-8. Nevertheless, there are cases of deaths from SARS-CoV-2 in various parts of the world that are not explained by the presence of these factors, so it is not yet clear what other factors could contribute to the development of more severe symptoms and the need for hospitalization. Some researchers propose, for example, that one of the causes of severe cases with no apparent explanation could be genetic predispositions linked to the gene encoding the cell-surface protein angiotensin-converting enzyme 2 (ACE2), used by SARS-CoV-2 to enter airway cells9. Our group was interested in investigating other variables that could explain such cases.
Because the pandemic caused by SARS-CoV-2 began during the influenza season, which is also the season with the highest incidence of various other respiratory viruses, we decided to evaluate whether the severity of confirmed cases of COVID-19 could be related to cases of coinfection.