Let’s imagine a booster dose would make the vaccine work just as well against the delta variant as it did in the original trials (95 percent effectiveness). Then let’s imagine we can find and give all 164 million fully vaccinated people in the U.S. a booster dose. This hypothetical scenario gives us an Re of about 4—still higher than the nonvariant virus’s R0 of 3. This tells us that boosters are not a great solution. Sure, they could bring the Re down a little, but even if every vaccinated person went and got a booster, we’d still be facing a big problem. We can do better.

Next, let’s imagine that, instead of a booster, we persuade more people to get vaccinated. If 75 percent of our population were vaccinated (another 80 million people), the Re would drop to just below 3. At that level, we know that our other public health measures (masking, business closures, etc.) should be enough to control the virus because the measures we took before the vaccines were available did reduce the spread of COVID whenever they were implemented.

Maybe you think convincing 80 million more people in the U.S. to get vaccinated is a big stretch? Let’s aim lower then. If we could make sure that the roughly 30 million people who have already received one dose of an mRNA vaccine get their second dose, then we would be up to 60 percent fully vaccinated. That would put our Re number for the delta variant down to 4—the same level of benefit as getting boosters to 180 million people!