- A new study from the CDC is some of the first evidence we have that vaccinated people don't transmit COVID-19 often.
- The study included nearly 4,000 healthcare and frontline workers who had mRNA shots from Pfizer or Moderna.
- The shots were 80% effective 2 weeks after the first dose, and 90% effective 2 weeks after both.
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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 the asymptomatic cases.
The CDC study, released March 29 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 significant protection against COVID-19 – around 90%.
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, shortly before the study was released.
"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."
Some of the first data that shows these vaccines work very well, even against asymptomatic infections
Researchers from the CDC looked at 3,950 healthcare employees and frontline workers who self-tested weekly (via nasal swabs) for COVID-19 between December 2020 and March 2021.
They found that two weeks after the first shot, participants were 80% protected from infections. Two weeks after the second shot, that coverage soared further to 90%. The finding mirrors what was seen in the clinical trials studying these two vaccines, which included tens of thousands of volunteer shot-takers around the world, last year.
The new data also echoes what has been seen in other countries: that mRNA vaccine protection ramps up over time, and is well boosted by the second shot. Experts stress it's still unclear how durable the 80% level of protection from a first shot would be without the second booster dose, which is why it's important to get a second shot on time.
In Israel, one shot of Pfizer's mRNA vaccine was shown to be 46% effective at preventing infections two weeks after the first dose, while two shots were 92% protective, when given a week to take effect. In the UK, one shot was shown to be 70% effective 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 data we have that the new mRNA vaccines from Pfizer and Moderna prevent all kinds of infections, even the 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, they help protect those around them from infections, too.
Stamping out asymptomatic infections, too
Previous studies found similarly small rates of infection among vaccinated people: 0.04% of vaccinated people tested positive for COVID-19 in a study from UCLA and San Diego Health, and 0.2% of vaccinated people tested positive in a similar study from the University of Texas Medical Center. In Minnesota, approximately 0.01% of fully vaccinated people have gotten COVID-19 so far.
But the new study went a step beyond others to date, by testing everyone in it every week, regardless of whether or not they had any COVID-19 symptoms.
The study showed that while asymptomatic infections are rare (meaning it's unlikely that vaccinated people will spread much virus to their neighbors), most people who do get sick tend to test positive for the virus before they know it, which is one key reason why this pandemic has been so tricky to combat.