During the Supreme Court’s hearing on the validity of COVID-19 vaccine mandates, Trump-appointed Justice Neil Gorsuch directed a question at the attorneys concerning why the U.S. doesn’t have similar mandates for flu vaccines. “We have flu vaccines,” said Gorsuch. “Flu kills, I believe, hundreds of thousands of people every year.”

When it was pointed out that this is an order of magnitude off from the real impact of flu, the Supreme Court ushered out a face-saving revised transcript, omitting the “of” so that Gorsuch said “hundreds, thousands” rather than “hundreds of thousands.” That’s not what he said. Even in the audio of this article supposedly debunking the idea that Gorsuch made such a claim, it’s obvious that “hundreds of thousands” is exactly what he said.

But apart from Gorsuch’s statement, claims about the deadliness of the flu have been raised by right-wing media and politicians throughout the COVID-19 pandemic. The truth is that flu kills only a small fraction of the number of people who have died from COVID-19 in each year of the pandemic.

However, flu does kill far more people than some diseases, like chickenpox, for which we do mandate vaccines. So why doesn’t the U.S. require mandatory flu vaccinations?

In 1952, the United States suffered its worst-ever outbreak of polio. In that year, 3,145 people died and another 21,269 suffered some level of paralysis. The result was a terror that saw public facilities closed across the nation. In my home town, the public pool was actually destroyed rather than take the chance that children might use it. 

Compare that with this chart of flu deaths over the last decade.

Flu deaths from 2011 to 2020

The paralysis associated with polio obviously inflated the fear, but flu also has long-term effects, also known as sequelae. That includes heart damage, brain damage, kidney failure … it’s a long list. So why did one disease end with a nationally mandated vaccine, while the other goes on and on with no such mandate?

The truth is, that there are mandates for flu vaccines, Many healthcare systems, and even companies, require their employees to get an annual vaccination for flu. An even larger number of companies, schools, and organizations encourage vaccination by fully covering the cost of vaccination (in areas where it’s not already free) and sometimes offering a bonus for getting that seasonal flu jab. But when it comes to the list of mandatory vaccinations required by schools, or the federal government, in most cases flu simply isn’t on the list.

The answer comes down to just one word: effectiveness.

Over the last 20 years, flu deaths have averaged around 30,000 a year. Over that same period, flu vaccines are estimated to have been 60% effective in the very best year. In the worst season (2003-2004) effectiveness was rated at just 10%. Over the whole period, flu vaccines were about 32% effective.

Compare that to the 90-95% effectiveness of COVID-19 vaccines on their release, or even their 80% effectiveness against omicron. The mumps vaccine is about 88% effective. The chickenpox vaccine is over 90% effective. The measles vaccine is about 93% effective. The polio vaccine is 99% to 100% effective. The smallpox vaccine scored 95%, and that (along with the fact that smallpox infected only humans) was enough to wipe the virus from the planet.

Unfortunately, flu vaccines also aren’t that great at mitigating the worst outcomes. In a 2021 study, the data analysis found that flu vaccines were “not significantly associated with receiving a clinical diagnosis of pneumonia.” However, they did find a 26% reduction in odds of ICU admission and “a  31% reduced risk of death compared with unvaccinated patients.” Those numbers are far, far below the improvements found with COVID-19, where vaccinated patients were over 90% less likely to die. More recent data has indicated that vaccinated and boosted people were 17 times less likely to be hospitalized than those who hadn’t been vaccinated.

That doesn’t mean getting a flu vaccine is pointless. Far from it. Flu vaccines are extremely important, especially for individuals who are at risk from the kind of damage flu can bring. And, because flu is much less communicable than COVID-19, with an R0 less than 2, a high level of vaccine use can still slow community spread, especially in years when there is a good match between vaccines and the type of flu that is circulating.

And that’s the other piece of this puzzle.

Flu viruses possess a kind of tinker-toy kit with a set of interchangeable proteins that can be expressed on the outer shell of the virus. That’s why flu viruses are often described using something like H1N1, where H1 stands for a particular hemagglutinin protein and N1 strands represent a specific form of neuraminidase. In 2020, the most common forms of flu were in the families of H1N1 and H3N2.

People exposed to one specific form of flu, or the vaccine for that specific form, have some temporary level of immunity to that type of flu, but just as we’ve seen with COVID-19 vaccines, that immune response declines over time. So each year, the flu that circulates is the result of a kind of Venn diagram where the common forms are an overlap of “what combo hasn’t been seen in a long time” and “what’s already out there in low numbers.” 

That’s why scientists so closely monitor things like a sudden jump of bird flu to people in some distant market, or an increase in a flu that’s shared with pigs in someplace closer to home. It’s why they’re particularly worried when they see some combination appear that’s been associated with a bad outbreak in the past, and that outbreak is far enough back to mean the population won’t be carrying much in the form of residual immunity.

In the end, COVID-19 vaccines are much more effective than flu vaccines. Their value, both to individuals and as a means of controlling community spread, is much higher. There’s a reason that the National Highway Safety Administration mandates seat belts and airbags: They work. That’s why we have mandates for mumps vaccines, measles vaccines, polio vaccines, and COVID-19 vaccines. They work.

Flu vaccines work. If the U.S. were to get 100% vaccinated for flu in any given year, the number of lives saved would almost certainly exceed the number lost in that worst year of polio. But until flu vaccines work better, a mandate is unlikely.