Probably the flu vaccine.
The first flu vaccine was developed in 1937, it only protected against one strain of the virus (we now vaccinate against three or four), but by 1942, another strain was added. The US Army even carried out the trials to test the vaccine and used it during WW2.
Was it effective? Yep, at least in 1944.
During the 1943–1944 season when the epidemic started in early November, the trial was repeated and 6,263 subjects were vaccinated. This time the results showed that only 2.2% of the vaccinated subjects had clinically assessed influenza disease compared with 7.1% of those not vaccinated, an efficacy of 69%.
That's a really good number for a flu vaccine. Better than we had the Achievements and challenges in antiviral drug discovery, which varied between 10 and 60 percent - flu vaccine effectiveness varies wildly because what specific strains are vaccinated against need to be picked ahead of time and there's no guarantee that these are going to be the most widespread in the next season.
1957 saw a flu pandemic of the Asian flu.
The rapid development of avaccine against the H2N2 virus and the availability of antibiotics to treat secondary infections limited the spread and mortality of the pandemic
It contrast, antivirals (against any virus) weren't discovered until the 50s and it was only in the late 60s that one effective against influenza started to be used. They are also used for treatment, not as mass prevention.
The Evolving History of Influenza Viruses and Influenza Vaccines
Achievements and challenges in antiviral drug discovery (full text PDF available)
Asian flu pandemic of 1957