But it is just a product of a little-understood phenomenon called “viral interference”: while one of these viruses holds sway in a person, or the population, for some reason the others can’t get a toehold. In September 2009, the swine flu pandemic that went on to sweep the world should have invaded Europe from the Americas. But the annual rhinovirus epidemic actually kept it at bay. The highly contagious new flu took over only when rhinovirus subsided, bumping RSV down the queue: RSV moved in only after that first wave of flu subsided.
The question now is where Covid-19 is going to fit amid this viral jostling. Not every virus takes turns like this, says Ian MacKay of the University of Queensland. Sometimes you can be infected by two at once. So which kind is Covid-19?
We do know it can infect someone alongside flu: the first Covid-19 case to die outside China was a 44-year-old man in the Philippines, who also had flu. We don’t know for sure that having flu at the same time makes Covid worse, but the fact that the Filipino victim was fairly young is worrying, says Florian Krammer of Mount Sinai School of Medicine in New York. “We assume the outcome of co-infection is not great.”