The news comes just in time for the 2020-2021 flu season, which threatens to coincide with a new upswing of Covid-19 infections in countries this fall and winter. The confluence of these two viruses could overwhelm already strained medical resources and complicate testing procedures, particularly given that the illnesses they lead to have remarkably similar symptoms.

A previous study, published in the Lancet in June, proved that a single dose of baloxavir was just as effective as Tamiflu (oseltamivir) at shortening the duration of the flu when administered twice daily for five days. But the newer July study is perhaps even more significant, as it shows the role that baloxavir plays in reducing the spread of illness among household members.

In 2018, baloxavir was initially approved for treating uncomplicated influenza A and B in Japan and treating people 12 years and older in the US. The July study was conducted during the 2018-2019 influenza season in Japan and assessed the ability of baloxavir to reduce transmission within a household. In the majority of cases, the first member of the household to contract influenza was a child 11 years old or younger. Notably, household members were unvaccinated for influenza. Upon the detection of a flu infection in at least one family member — judged both by fever, at least one moderate or worse respiratory symptom, and a positive flu test — the uninfected members of the households were either given a placebo or baloxavir.

In households that received the placebo, 13.6% of the residents contracted influenza. Only 1.9% of those that received baloxavir came down with the flu, and there were no serious side effects noted in the treatment group.

But there are several caveats. Two strains of influenza typically circulate in the fall and winter months, influenza A and B. Although baloxavir is approved for use against both influenza A and B, influenza B was not active in Japan during the 2018-2019 flu season. The hope, though unproven in this study, is that the drug also protects against infection by influenza B.

The success of baloxavir against the transmission of influenza is significant, and provides hope that drugs directed against other viral enzymes could be developed to treat and prevent Covid-19.

In the critical condition many health care systems find themselves in today — with hospitals and providers already overwhelmed after months of treating Covid-19 infections — an approved drug that could help reduce the number of people falling ill from the flu this winter will be a welcome relief. It is unlikely that effective antiviral Covid-19 drugs will be available for prophylactic treatment for the majority of those at risk for Covid-19 by this fall and winter. Similarly, a vaccine that is both safe and effective may also not be widely available.

A combination of a seasonal flu vaccine and an effective prophylactic drug such a baloxavir — in addition to personal protective behaviors already being practiced across the US — could significantly reduce the severity of the coming influenza epidemic and spare our hospitals the worst of the combined effects of simultaneous epidemics.