COVID isn’t the first time a vaccine was rapidly developed to fight a disease. All precious pandemics were referred to as “flu” variants. You probably know about 1918, but we had flu pandemics in 1957-1958, 1968, and 2009.
This CDC website has descriptions of each along with what age group was targeted: https://www.cdc.gov/flu/pandemic-resources/2009-h1n1-pandemic.html
The really cool site is also on CDC.gov. Storybook posted stories from people who lived through pandemics. See those here: https://www.cdc.gov/publications/panflu/index.html
If you aren’t familiar with Storybook, they document first hand stories in order to preserve history. I know they have a huge project taping WWII veterans.
If you can find a few minutes to read the really short stories it will prove we’ve been here before. China has been the source before too. Even when the young were targeted we never shut done the country.
This story shows what an American Indian family did for themselves in 1918, long before modern medicine existed. Many of them were sick, all of them survived: https://www.cdc.gov/publications/panflu/stories/cure_janis.html