The severity of the 2017/2018 flu outbreak continues to increase in the US, UK, and Australia as we muddle through the second half of the flu season.

As with previous years, there are several different strains currently circulating that have evolved to be dangerous to humans.

According to the Centers for Disease Control and Prevention (CDC), most of the cases seen at hospitals thus far have been from the influenza type A H3N2 virus, though a few (A)H1N1 – the infamous swine flu – and some less virulent type B strains have also been reported. In Australia, an extreme version of the H3N2 virus has already infected more than 2.5 times as many people as were affected during the entirety of the 2016/2017 season, and over 300 have died. 

Now, following an alert from the Chinese Centre for Health Protection (CHP) in Hong Kong, it appears there is a new strain we must keep an eye on: H7N4.

A woman in the Chinese city of Changzhou was hospitalized on December 25, 2017, after contracting the first ever human infection from this A-type influenza, previously known to only infect birds. The 68-year-old was apparently in contact with live poultry before she fell ill. Details on her symptoms were not available, but the statement notes that she was discharged on January 22. Officials are on high alert for new cases, yet none have been identified thus far.

“The CHP's Port Health Office conducts health surveillance measures at all boundary control points. Thermal imaging systems are in place for body temperature checks on inbound travelers,” the agency stated. “Suspected cases will be immediately referred to public hospitals for follow-up.”

Of the estimated 144 existing influenza A strains, some can opportunistically invade the immune systems of both birds and mammals, whereas others are limited to only one animal class or even a small subset of species within that class. Every so often, however, viruses acquire the ability to infect a new organism. This evolutionary event, called viral host switching, can ultimately lead to the emergence of a pandemic within its newly affected species. Human diseases that arose in this fashion include HIV, Ebola, and SARS.

But before a virus can reach that level of devastation, it must gain the additional ability to readily spread between members of its new host species. Many avian strains of H5, H9, and H7 lineages are known to rarely infect people, but only after being transmitted from a bird host. Transmission from person to person remains very rare, meaning H7N4 and its family members are unable to wreak global havoc – for now.  

Meanwhile, health officials are grimly monitoring the spread of H3N2.

“In the past five seasons, influenza-like illness has been elevated for between 11 and 20 weeks, and we’re only at week 11 now, so we could potentially see several more weeks of activity,” said CDC Acting Director Dr Anne Schuchat at a February 9 press briefing.

"Levels of influenza-like illness across the country are now as high as we observed at the peak of the 2009 H1N1 pandemic. This doesn’t mean that we are having a pandemic, just that levels of influenza-like illness are as high as what we saw during the peak of H1N1. In addition, overall hospitalizations are now significantly higher than what we’ve seen for this time of year since our current tracking system began almost a decade ago."