To answer this question, the Healthline website has published a review of a study by the Centers for Disease Control and Prevention (CDC).
In an April 2 the CDC studied almost 4,000 vaccinated healthcare personnel, first responders, and other essential and frontline workers.
The COVID-19 vaccines produced by Pfizer and Moderna are highly effective at preventing COVID-19 cases in real-world conditions, and research suggests they should maintain their effectiveness over time. What remains unclear, however, is exactly how long the vaccines prevent COVID-19, if booster shots may be needed down the road, or if vaccines will need to be tweaked to fight against emerging variants of the virus.
They found that the messenger RNA (mRNA) vaccines developed by pharmaceutical companies Pfizer-BioNTech and Moderna prevented 80 percent of cases after the first dose and 90 percent after the second dose. The frontline workers in the study were tested for COVID-19 every week for 13 weeks.
Researchers said the dearth of positive COVID-19 tests in the study group indicates that the vaccines reduce the risk of transmission of SARS-CoV-2 by vaccinated individuals to others. “Reducing the risk for transmissible infection, which can occur among persons with asymptomatic infection or among persons several days before symptoms onset, is especially important among healthcare personnel, first responders, and other essential and frontline workers given their potential to transmit the virus through frequent close contact with patients and the public,” the report noted. “There’s more and more evidence showing that… the transmission of the virus after vaccination is likely very low,” Dr. Susan Bailey, an allergist and immunologist and president of the American Medical Association, told Healthline.
Separately, Pfizer-BioNTech said that the ongoing phase 3 clinical trial of its mRNA vaccine shows that strong immunization persists for at least 6 months among vaccinated individuals. Researchers found that the vaccine was 100 percent effective against severe disease as defined by the CDC, and 95.3 percent effective against severe COVID-19 as defined by the Food and Drug Administration (FDA).
The vaccine was also found to be 100 percent effective against one of the main COVID-19 variants (known as B.1.351) currently circulating widely in South Africa. A study that included 12,000 vaccinated individuals also found “no serious safety concerns” with the Pfizer-BioNTech vaccine, the companies announced. “The good news is that in the 6-month status report from Pfizer, immunity stays very strong, and we anticipate that it will continue to stay strong,” said Bailey. “These people [in the study] have had the vaccine the longest, and it tells us it lasts at least 6 months,” added Bailey.
“But it’s definitely longer than that — it’s not just going to drop off after 6 months. I would have been concerned if efficacy had dropped by a third or half.” The fact that COVID-19 vaccine effectiveness remained almost unchanged over the span of the study period is an indication that protection will be enduring. Bailey noted that some vaccines, such as those for measles, mumps, and rubella, generally confer lifetime immunity. Others, such as the flu vaccine, require a new shot every year.
Protection lasts for at least 6 months, likely longer.