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, of whom nearly 3,000 received a vaccine.
The shots were 80% effective two weeks after the first dose and 90% effective two 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 asymptomatic cases.
The CDC study, released Monday in the Morbidity and Mortality Weekly Report, featured data from nearly 4,000 healthcare workers, first responders, and essential workers across six US states. Nearly 3,000 of them received a COVID-19 vaccine between December and March. Everyone in the study self-tested weekly for any infections.
Two weeks after receiving the first shot of a COVID-19 vaccine from Pfizer or Moderna, antibody protection in this group was already ramping up, and the risk of infection was slashed by about 80%.
But getting both vaccine doses (and then allowing those shots two more weeks to take effect) protected the frontline workers in the study even better – making them about 90% less likely to catch COVID-19, despite their continued, consistent exposure to the coronavirus on the job.
What’s more, vaccinated people in the CDC study rarely caught asymptomatic infections.
“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 COVID-19 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 14, 2020 to March 15, 2021.
Of those, 2,479 received two doses of a COVID-19 vaccine, 477 had received only one dose by the end of the study, and the rest were not vaccinated.
The researchers found that two weeks after getting a first shot, participants’ risk of infection was slashed by 80%. Two weeks after the second shot, that number soared to 90%.
These real-world findings mirrored earlier results from Pfizer and Moderna’s clinical trials last year, which included tens of thousands of volunteers around the world, and showed that mRNA vaccine protection begins ramping up in people just two weeks after a first shot, when their infection rates are compared to their unvaccinated peers.
The new data also echoed recent observations in other countries that mRNA vaccine protection ramps up over time, and is well boosted by a second shot. (Or, a first shot, in the case of people who’ve already had a COVID-19 infection.)
Experts have stressed that it’s still unclear how durable the 80% level of protection from a first shot (or, exposure) 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 anywhere from 46-60% effective at preventing infection two weeks later, while two shots were found to be 92% protective when given a full week to take effect. In the UK, one shot of Pfizer 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, which is great news for everyone
This CDC study is 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 the asymptomatic ones that could spread from vaccinated people to vulnerable others without notice.
The data confirms what many scientists have long suspected: COVID-19 vaccines don’t just protect the vaccinated, but can also 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
During the study, only eight partially vaccinated people tested positive for COVID-19, and only three fully immunized people did (for comparison, there were 161 COVID-19 infections in the unvaccinated study group).
Previous studies of authorized COVID-19 vaccines have found similarly small rates of infections among vaccinated people, so-called “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 in it 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 these “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.
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