If vaccines for the flu and COVID-19 were competing as wrestlers in a ring, the COVID-19 shot would have the flu shot pinned flat against the mat.
The efficacy of influenza vaccines year-to-year can range between 20% and about 70%. But an immunology expert notes COVID-19 vaccines approved for use in the U.S. have efficacy rates in the 90% range.
What does vaccine efficacy actually mean? It can mean a lot of different things according to Andrew Pekosz, a professor of microbiology at Johns Hopkins Bloomberg School of Public Health.
He explains how various people might measure vaccine efficacy:
- Reductions in hospitalizations and deaths
- Reductions in infections that show symptoms
- Numbers of asymptomatic infections
“But, if you ask, ‘what are vaccines that protect against symptomatic disease,’ this is where the mRNA vaccines for COVID-19 really hit that 90% efficacy level,” Pekosz said. “Meaning that out of 100 vaccinated people, 90 of them will be protected from SARS-CoV-2 symptomatic infection.”
Those 10 infected people, however, will suffer much weaker intensity of symptoms and may get infected without knowing it, because immunity offered by the vaccine does protect them to some degree.
“You also see vaccine efficacy can sometimes be highest in protecting against severe disease — because even the slightest head start that your immune system can get to make antibodies and other responses to an infection can really change the course of infection and prevent severe disease,” Pekosz said.
For comparison purposes, Pekosz notes other vaccines have success rates similar to those for COVID-19. He calls chickenpox and measles vaccines, for example, “very efficacious.”
“Meaning that sometimes after one or two boosts you can also hit that 90% level of protecting against infections,” Pekosz said. “That’s an ideal place to be — particularly for infections like measles, mumps, rubella; there, you really want the strongest amount of protection because the severity of disease can really bring with it a lot of bad outcomes.”
While a few vaccine ‘breakthroughs’ — cases of fully vaccinated people getting sick — are expected, Pekosz said the share of breakthroughs exceeding what was seen during clinical trials would suggest either a problem with the vaccine, or that the virus itself has changed.
Influenza viruses change every year, and flu vaccines don’t offer long-term immunity. That’s why yearly flu shots are recommended. With COVID-19 vaccines, it’s unclear what the future will hold.
Pekosz said the virus that causes COVID-19, SARS-CoV-2, has yet to change in a way that would allow it to significantly evade current vaccines. There is some real-world evidence that a South African variant of the virus may lead to a higher proportion of breakthroughs among people exposed to it — but even then, the number of people fully protected remains high.
The immune response longevity from mRNA COVID-19 vaccines are still being researched, as is whether another booster could someday be needed.
“We already know that after six months, we’ve had a very strong response in most people to vaccination. We’ll have to see if that stretches to 12 months, or even longer,” he said. “So, it’s that combination of, how long will the vaccine immunity last, and how will the virus change, that are sort of the wild cards right now when we think about the COVID-19 vaccines.”
