What does a “normal” flu season look like?
Flu season usually peaks between December and February. Influenza typically causes about 45 million illnesses, 810,000 hospitalizations, and 61,000 deaths in the United States each year, according to the U.S. Centers for Disease Control and Prevention.
Why is this season different?
Though caused by a different virus from the one that causes COVID-19, the flu is also a respiratory viral disease, so everything we are doing to slow transmission of COVID-19, such as wearing face masks, frequent hand-washing, and physical distancing, should also reduce transmission of flu.
How can there be so many cases of one respiratory illness and so few of another? Are the flu numbers really that low?
We don’t track the flu the way we track COVID-19. The average seasonal flu exists in a weird liminal space, serious enough to keep an eye on, but also not so serious that we are literally trying to count every single case. After all, most people who get sick with the flu won’t even bother to go get tested for it. They’ll have a couple of bad days in bed (if they’re able to take off work) and otherwise, go about life basically unchanged.
Instead, counts of flu cases come from a few different surveillance systems, including a network of around 100 public health and 300 clinical labs that participate in virologic surveillance, reporting weekly test numbers and positive cases to the CDC. There’s also a network of doctors’ offices reporting cases of “influenza-like illness,” a network of hospitals reporting lab-confirmed cases, and mortality surveillance data from the National Center for Health Statistics, which pulls numbers for flu, pneumonia, and other respiratory illnesses directly from death certificates.
This is why annual numbers of flu cases (and even deaths) are an estimate, extrapolated from the narrower picture we can see.
Why did COVID restrictions work so well against the flu?
Influenza doesn't spread as well as the coronavirus. The average number of people one person with the flu infects - a measure known as the reproductive number - is 1.28. Typically, someone sick with the coronavirus passes it to between 2 and 2.5 people. In part, this difference is because the coronavirus can be airborne, remaining suspended in the air for hours. That's not true of the flu, though viruses can jump from person to person via respiratory droplets and contaminated surfaces.
The coronavirus' higher reproductive number means it is more challenging to prevent transmission through non-pharmaceutical interventions than it is for influenza.
Could this year’s flu season increase the severity next year?
Because of the current restrictions and precautions, everyone is taking this season, far fewer people will be infected or exposed to the flu virus, and therefore won't become immune to certain strains of the virus. So the number of people who may have more severe infections next year is likely to be greater because immunity will be lower.
Should you still get the flu shot this year?
Yes! Despite the low flu numbers this season, you should still take steps to protect yourself, including getting a flu shot.
The flu vaccine takes about two weeks for antibodies to develop and begin protecting you from the illness, so getting the flu vaccine any time through mid-April will still be helpful in preventing the flu. However, if you’re getting the COVID-19 vaccine, 14 days must elapse between getting the COVID-19 shot and any other vaccinations, including the flu vaccine.
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