A Californian Covid-19 patient has also tested positive for flu, in what may be America's first instance this season of someone catching both infections.
The double infection in Solano County prompted health officials to again urge people to get flu shots amid fears that colliding waves of flu and Covid will put health systems around the world under intense pressure.
Social distancing and mask wearing precautions to curb Covid-19 are thought to have had the knock-on effect of stifling the flu season in the southern hemisphere's winter.
America's Centers for Disease Control and Prevention reported recently that the flu season came and went in countries like Australia, Chile and South Africa with "virtually no influenza circulation".
Flu surveillance monitoring shows that going into the northern hemisphere's normal flu season, levels of the infection also remain low in countries including America and in the UK.
Yet if the seasonal flu picks up as America and Europe face a second wave of coronavirus then health officials fear a “twindemic” that will overwhelm wards with huge numbers of sick. Patients hit by both viruses could also develop more severe illness.
Little is known about the potential risks to patients who have both infections, the Washington Post reported, because very few were recognised in the early stages of the pandemic.
A survey of patients presented last week to the Infectious Diseases Society of America found those with both bugs did not get sicker, but the sample was only 18 people.
Yet a study of 64 co-infections in China's city of Wuhan found those who had both infections took longer to get better and had to stay in hospital longer.
Bela Matyas, the Solano County health officer, said the Californian patient worked in healthcare and had subsequently recovered.
“This is a very clear indication of the potential for this to occur,” he told the San Francisco Chronicle. “We now have flu in our community at the same time we have Covid ... Contracting either disease may weaken your immune system and make you more susceptible to the other disease.”
“It certainly can’t be good to be infected with both and it may well be a greater challenge to the person who has both infections, and that could make the outcomes worse,” said John Swartzberg, an infectious disease specialist at the UC Berkeley School of Public Health. “So we should do everything we can to prevent infections.”
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