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AMERICA IS IN the throes of its worst flu season in at least seven years. The Centres for Disease Control and Prevention estimates that some 29m Americans have been infected since October—20% more than at the same point last year. For the first time since 2020, the flu is leading to more hospitalisations and deaths than covid-19 (see chart 1).
Most American states are being hit equally hard (with the exception of remote places such as Alaska and Hawaii). Hospitals in Europe have also reported one of the worst outbreaks in years. However, in the southern hemisphere, where the flu season runs from April to the end of September, there was no evidence that this year’s strains were any more severe than usual.
Only a fraction of flu cases are actually confirmed with a test, which makes an outbreak difficult to monitor. At the start of February hospitalisation rates for confirmed cases in America were 44% higher than during last year’s peak. Hospitalisations for all “influenza-like illnesses” (ie, those with the same symptoms as the flu but without a confirmed test result) are at their highest since at least 1997. Patients with these symptoms are also making up a higher share of doctor visits. During the week beginning February 3rd, such illnesses accounted for 7.8% of visits to general practitioners in America—the second time the share has exceeded 7.5% this season, but only the sixth time since 2000, including over the pandemic.
Children, pregnant women, old people and those with weakened immune systems are the most vulnerable to serious complications from the flu. The hospital data show that Americans aged over 65 in particular are being more seriously affected than usual (see chart 2). During the worst weeks of last year’s flu season, they were hospitalised at 4.5 times the rate of those aged 50-64. This year that ratio has risen to six.
Experts cannot say with certainty why this season has been so severe. Some cyclical variation is to be expected: a bad season often follows a few milder ones. Covid-era lockdowns and mask-wearing have meant that fewer people have been exposed to the virus in recent years.
Falling vaccination rates don’t help. By this point in the 2019-20 season roughly 69% of American children between the ages of six months and four years had been vaccinated. This season only around 54% have been. For pregnant women the vaccination rate has dropped from 55% to 37% in the same period; for those aged 65 and over, from 51% to 43%.
At the same time, early data from those who have been vaccinated suggest that this year’s flu shots may have been less effective than usual. Evidence from the southern hemisphere suggests that they reduced hospitalisations among high-risk groups by 35%, compared with 50% the year before. (Vaccines require scientists to predict the most dominant strains ahead of each flu season.)
America’s flu season typically ends in February. But hospitals are still under strain: when your correspondent spoke to an administrator at the Massachusetts General Hospital in Boston on February 19th, she described the emergency room as “out of control”. ■