As flu season inches closer, scientists have outlined best defense against influenza from getting it worse, which may also protect against the deadly coronavirus.

Dr Benjamin Singer, a Northwestern Medicine pulmonologist who treats COVID-19 patients in the intensive care unit, has described four factors that could determine the severity of the upcoming flu season.

Social distancing policies designed to limit the spread of COVID-19 are also effective against the flu.

"If COVID-19 cases begin to spike in the fall of 2020, re-tightening social distancing measures could help mitigate early spread of the flu to flatten the curves for both viruses," Singer said in an editorial that was published in the journal Science Advances.

"As we await vaccine trials for COVID-19, we should plan to increase rates of vaccination against the flu, particularly among older adults who are more susceptible to both the flu and COVID-19," added Singer, an assistant professor of pulmonary and critical care and biochemistry and molecular genetics at Northwestern University Feinberg School of Medicine.

The world needs widespread availability of rapid diagnostics for COVID-19 and other respiratory pathogens "because co-infection with another respiratory pathogen, including the flu, occurred in more than 20 per cent of COVID-19-positive patients who presented with a respiratory viral syndrome early in the pandemic".

The fourth factor is to galvanize public health efforts aimed to limit viral spread, increase vaccination rates, deploy rapid diagnostics and expand other health care services for vulnerable populations, including communities of colour, the poor and older adults.

"The COVID-19 pandemic has highlighted unconscionable disparities among African Americans, Latinx and Native Americans," said Singer.

The Centers for Disease Control and Prevention (CDC) estimated that the 2019-2020 seasonal influenza epidemic resulted in tens of millions of cases and tens of thousands of deaths.

"Even in non-pandemic years, the flu and other causes of pneumonia represent the eighth-leading cause of death in the United States, and respiratory viruses are the most commonly identified pathogens among hospitalized patients with community-acquired pneumonia," Singer noted.