CDC: Real-world data shows Pfizer's and Moderna's COVID-19 vaccines are 90% effective at preventing infection

• A study from the CDC provides early evidence that vaccinated people don't often transmit COVID-19.
• It included nearly 4,000 healthcare and frontline workers who got mRNA shots from Pfizer or Moderna.
• The shots were 80% effective two weeks after the first dose and 90% effective two weeks after both.

On Monday, the Centers for Disease Control and Prevention released some of the first real-world evidence that mRNA vaccines are 90% effective at preventing COVID-19 infections, even asymptomatic cases.

The CDC study, released Monday in the Morbidity and Mortality Weekly Report, featured data from nearly 4,000 first responders and frontline workers in the US who received the vaccines between December and March and self-tested weekly for infection after their vaccinations.

The new study found that two weeks after receiving the first shot of a COVID-19 vaccine from Pfizer or Moderna, recipients were about 80% protected from infections. Getting both doses offered about 90% protection against COVID-19.

What's more, vaccinated people rarely transmitted the virus asymptomatically to others.

"This study shows that our national vaccination efforts are working," CDC Director Rochelle Walensky said in a statement.

"These findings should offer hope to the millions of Americans receiving COVID-19 vaccines each day and to those who will have the opportunity to roll up their sleeves and get vaccinated in the weeks ahead. The authorized vaccines are the key tool that will help bring an end to this devastating pandemic."

It's some of the first data indicating these vaccines work very well even against asymptomatic cases

Researchers from the CDC looked at 3,950 healthcare employees and frontline workers who self-tested weekly (via nasal swabs) for COVID-19 from December to March.

They found that two weeks after getting the first shot, participants were 80% protected from infection. Two weeks after the second shot, that coverage soared to 90%. The finding mirrored results in the vaccines' clinical trials, which included tens of thousands of volunteer shot-takers around the world last year.

The new data also echoed observations in other countries that mRNA vaccine protection ramps up over time and is well boosted by the second shot. Experts have stressed that it's still unclear how durable the 80% level of protection from a first shot would be without the booster dose, which is why it's important to get the second shot on time.

In Israel, one shot of Pfizer's mRNA vaccine was found to be 46% effective at preventing infection two weeks later, while two shots were found to be 92% protective when given a week to take effect. In the UK, one shot was found to be 70% effective at preventing symptomatic infections in healthcare workers, while two shots were 85% effective.

The vaccines appear to limit transmission

The new CDC study is also some of the first real-world data we have indicating that the new mRNA vaccines from Pfizer and Moderna prevent all kinds of infections, even asymptomatic ones that can spread to vulnerable people without notice.

It confirms what many scientists have long suspected: COVID-19 vaccines don't just protect the vaccinated but help to protect those around them from infections, by limiting viral spread community-wide.

Breakthrough infections in vaccinated people are rare, but they do happen

Previous studies of authorized COVID-19 vaccines have found similarly small rates of infections among vaccinated people, or "breakthrough" infections.

In a study from UCLA and San Diego Health, 0.04% of vaccinated people tested positive for COVID-19. In a similar study from the University of Texas Southwestern Medical Center, 0.2% of vaccinated people tested positive. In Minnesota, about 0.01% of fully vaccinated people have gotten COVID-19 so far.

But the new CDC study went a step beyond the others by testing everyone every week, regardless of whether they had any COVID-19 symptoms.

The study found that people who do get sick with COVID-19 post-vaccination, in such breakthrough infections, tend to test positive for the virus before they show any symptoms or outward signs of illness - a key reason that the spread of this virus has been so tricky to combat, and that wearing a mask is still so vitally important.