The CDC report is significant, experts said, because it analyzed how well the vaccines worked among a diverse group of front-line working-age adults whose jobs make them more likely to be exposed to the virus and to spread it.
The workers came from eight locations in six states — Arizona, Florida, Minnesota, Oregon, Texas and Utah. They received vaccinations between mid-December, when the doses first became available, to mid-March, a 13-week period that included the deadly winter surge that was killing more than 3,000 people a day by January. The study is also one of the first to estimate vaccine effectiveness among participants against infection — rather than just monitoring for symptomatic cases — including infections that did not result in symptoms, according to the CDC.
Among 2,479 fully vaccinated people, just three had confirmed infections. Among 477 people who received one dose, eight infections were reported.
By comparison, among 994 people who were not vaccinated, 161 developed infections.
No deaths were reported.
CDC Director Rochelle Walensky said the study shows national vaccination efforts are working.
The authorized coronavirus vaccines “provided early, substantial real-world protection against infection for our nation’s health care personnel, first responders, and other frontline essential workers,” she said in a statement. “These findings should offer hope to the millions of Americans receiving coronavirus 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.”
The study is ongoing and researchers will share more details on the infections in people who were partly or fully vaccinated, known as “breakthrough infections.” Researchers are also studying whether people who became infected despite vaccination may have less severe or briefer illnesses and whether they shed a lower amount of virus for less time.
Infectious-disease and vaccine experts said the latest data proves encouraging.
“It’s not surprising, but it’s incredibly reassuring,” said Paul A. Offit, a vaccine expert at Children’s Hospital of Philadelphia who helps review vaccine safety and efficacy as an outside adviser to the Food and Drug Administration and was not involved in the CDC study. “It’s yet another reason to get vaccinated.”
The drug companies’ clinical trials of their coronavirus vaccines in 2020 were conducted when the United States was not experiencing the biggest surges in coronavirus cases, said Monica Gandhi, an infectious-diseases expert at the University of California at San Francisco.
“What we really want to do is test these vaccines in the real world, to see how well they work,” said Gandhi, who was not involved in the CDC study. That study took place during “one of the most scary, most horrible surges” in the United States, she said, describing it as an important stress test for the vaccines. The results, she said, show how “it’s really incredible news for how safe you can feel after vaccination.”
The Pfizer-BioNTech and Moderna vaccines, which require two doses, were more than 90 percent effective in preventing symptomatic covid-19 disease in clinical trials conducted before the vaccines received emergency use authorization in December from the FDA. The Pfizer-BioNTech vaccine requires special handling and must be kept at especially cold temperatures in special freezers long-term.
“There was no guarantee that the vaccines were going to perform the same in the real world, where there were a lot of concerns about freezers and temperatures,” said Mark G. Thompson, a CDC epidemiologist and lead author of the report. The clinical trials had large numbers of older people, but the CDC study focused more on working-age adults and the ability of the vaccines to protect against infection, regardless of whether symptoms developed.
“The big takeaway here is that starting 14 days after receiving both doses of these … vaccines, these front-line workers were 90 percent less likely to be infected with the virus that causes covid-19,” Thompson said.
Reducing risk for transmissible infection, which can occur several days before symptoms appear, is especially important for health-care personnel and other essential workers who may not know they are infected, he said.
About 72 percent of the participants were 18 to 49 years old and the majority were female. Most participants were White and healthy, with no chronic medical conditions. About half were health-care providers; 21 percent were firefighters, police and emergency medical technicians; nearly one-fourth were teachers, delivery workers and other essential personnel whose jobs required contact within three feet of the public or co-workers.
About 63 percent of participants received the Pfizer-BioNTech vaccine and 30 percent received the Moderna shot. Five people received the Johnson & Johnson vaccine. Researchers are trying to verify what vaccine was administered among the remaining participants. Most got their first dose during the last two weeks of December.
Participants conducted nasal swabs weekly for 13 weeks, regardless of whether they experienced symptoms. If they showed symptoms of covid-19-like illness, such as fever, chills, cough, shortness of breath or change in smell or taste, they collected an additional nasal swab and saliva sample. All specimens were sent to one lab in Marshfield, Wis.
Thompson said the study was by far the most complicated and ambitious to design in his more than 30 years of conducting field research. It was challenging to recruit thousands of participants from a variety of backgrounds in the context of widespread skepticism and lack of trust in science and research, he said. Researchers had never asked people to collect their own nasal swabs and ship saliva specimens, and it was unclear at the start whether busy adults working in front-line jobs would consistently respond to those tasks.
“But participants have been very dedicated and so far, on average, 80 to 100 percent of participants complete these tasks every week,” Thompson wrote in an email.
The CDC had to negotiate with FedEx and UPS to allow thousands of specimens to be shipped for testing, he said.
The study did not provide an estimate of the vaccines’ benefits against the virus variants circulating in the United States. Thompson, who is leading CDC’s evaluation of coronavirus vaccine effectiveness, said the agency will have more to say in about a month when scientists have completed genetic characterization of the collected virus specimens.
As the study progresses, scientists will also analyze the effectiveness of the specific coronavirus vaccines and their effectiveness in preventing symptomatic infections or severe outcomes, such as hospitalization.
Despite the strong protection afforded by the vaccines two weeks after the first dose, scientists are still trying to figure out how long someone is protected against the disease after being fully vaccinated and whether two doses provide longer-lasting protection than one dose.
Limitations of the study include the small number of confirmed infections and self-collection of specimens and delays in shipments to the lab that could reduce detection of the virus and result in overestimating vaccine effectiveness.
The latest results come as President Biden has set a new goal of administering 200 million doses in his first 100 days in office, which arrives April 30. The nation is poised to meet the revised target, as the seven-day average of daily vaccinations surpasses 2.5 million.
But infectious-disease experts are worried the pace needs to be faster to reach the high levels of immunity needed to slow the virus, especially as more transmissible variants spread throughout the country. To reach the level of protection needed, about 80 percent of the population has to be immunized, meaning that about 260 million people need to get vaccinated. That would require 3 million to 3.5 million shots being administered each day until April 30.