No one has bothered to answer the question why we never required masks in school to prevent the flu.
There's multiple reasons:
1) The data behind masking was significantly mixed and understudied prior to 2020. I've made this point a few times in different threads, and it's often an inconvenient truth to those who want to rewrite history. But there were very few studies prior to 2020 that showed a benefit to masking for the general public, including one study that suggested that
masking increased the risk of influenza infection. You can even review the CDC's recommendations during the
2009 H1N1 outbreak. The CDC clearly stated at the time: "Information on the effectiveness of facemasks and respirators for decreasing the risk of influenza infection in community settings is extremely limited." You can see what their recommendations were at the time following the evidence they had. Masks were certainly considered back then but not deemed an important tool for widespread use. Clearly the data from the past two years now suggests otherwise.
2) As pmv described, the infectivity rate or reproduction value (R value) is significantly lower for influenza. Seasonal influenza seasons are estimated to have a
R value of 1.28, while the 2009 H1N1 neared 1.46, while the 1918 Flu pandemic the R was closer to 1.8. For SARS-CoV-2, estimates put the
original strain around 2.87, with Delta and Omicron having higher
average R value ~5, with Omicron likely being higher. To put this in other terms, 5 transmission cycles from a single person would lead to ~8 people being infected with influenza (using the 2009 H1N1 value of 1.5), while for SARS-CoV-2, that number would be 243 (using a R value of 3). The levels of contagiousness are very different.
3) Influenza has been circulating in human populations for 100+ years.
Previous influenza infections can confer some level of protection to new strains and may explain why some strains of influenza end up affecting more younger adults rather than older adults. From what we know as of right now, previous infections with seasonal coronaviruses have not
provided significant cross protection. This means that the human population is immunologically better prepared for a influenza pandemic rather than a COVID-19 pandemic.
4) There have been longstanding and approved vaccines and pharmaceutical therapies available for influenza, with these approved down to age 6 months. These items were stockpiled in preparation for the 2009 H1N1 pandemic. The COVID-19 vaccines are far superior but newer, and still is lacking in data for children 6 months to 4.9 years of age. Most of the effective COVID-19 therapeutics are limited in utility with the best data supporting use only in hospitalized patients. Most of the monoclonals have been rendered ineffective because of omicron, and the "pill" antivirals are only coming online. Many of these newer therapies have been limited to patients older than age 12 (although one of the now obsolete monoclonals could be used under EUA in younger ages).
5) SARS-CoV-2 has been clearly implicated in causing MIS-C. Influenza has not and has not been implicated in any other related disease like Kawasaki Disease.
If a H5N1 strain of influenza or other high pathologic strain of influenza became a significant risk to human populations, I think universal masking would absolutely be part of the public health response. Thankfully we've already stockpiled vaccines in preparation of such an event.