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.