Pfizer's COVID-19 vaccine still effective after six months: study

How long does immunity last after vaccination? Scientists are a bit closer to answering that question

By Nicole Karlis

Senior Writer

Published April 2, 2021 4:14PM (EDT)

Medical syringes are seen with Pfizer company logo displayed on a screen in the background in this illustration photo taken in Poland on October 12, 2020. (Photo illustration by Jakub Porzycki/NurPhoto via Getty Images)
Medical syringes are seen with Pfizer company logo displayed on a screen in the background in this illustration photo taken in Poland on October 12, 2020. (Photo illustration by Jakub Porzycki/NurPhoto via Getty Images)

On Thursday, Pfizer released updated clinical trial data that revealed that six months after clinical trial volunteers were inoculated, its COVID-19 vaccine remains highly effective.

In a rush to bring a viable vaccine to the public, biopharmaceutical companies were not initially able to guarantee how long immunity against COVID-19, the disease caused by the coronavirus SARS-CoV-2, would last. Now, Pfizer is able to confirm that immunity lasts at least six months for its two-shot mRNA vaccine.

The question of how long SARS-CoV-2 vaccine immunity lasts has been a bit of a mystery. Those infected with COVID-19 only retain their immunity for 3 to 12 months, studies show — a condition known as "transient immunity," meaning temporary, as opposed to long-lasting "durable immunity." Vaccines can confer different types of immunity than a cleared infection, though because SARS-CoV-2 is a novel virus, scientists were uncertain how long vaccinated immunity would last. The new clinical trial data is encouraging for public health. 

The findings come from an ongoing review as to how volunteers from the vaccine's late-stage trial are doing, and whether they contracted COVID-19 with symptoms or not. The analysis examined the vaccine's efficacy in 46,307 people who enrolled in the Phase 3 trial, starting in July. Of the 927 cases of symptomatic COVID-19 cases from the clinical trial group, 850 of those cases came from people who received a placebo; 77 cases were from those who were vaccinated.

This means that the vaccine still has 91.3 percent vaccine efficacy rate up to six months after receiving the second shot, and 100 percent vaccine efficacy in preventing severe disease as defined by the U.S. Centers for Disease Control and Prevention (CDC). A further analysis found that the vaccine was also 100 percent effective in preventing COVID-19 cases in South Africa where the dangerous B.1.351 variant is now the dominant strain.

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"It is an important step to further confirm the strong efficacy and good safety data we have seen so far, especially in a longer-term follow-up," said Ugur Sahin, CEO and co-founder of BioNTech, in a press statement. "These data also provide the first clinical results that a vaccine can effectively protect against currently circulating variants, a critical factor to reach herd immunity and end this pandemic for the global population."

Notably, the data collected showed no serious safety concerns. Overall, adverse reactions in participants over the age of 16 include pain at the injection site (84.1%), fatigue (62.9%), headache (55.1%), muscle pain (38.3%), chills (31.9%), joint pain (23.6%), fever (14.2%), injection site swelling (10.5%), injection site redness (9.5%), nausea (1.1%), malaise (0.5%), and lymphadenopathy (0.3%).

In a separate announcement, Pfizer and BioNTech announced on Wednesday that the vaccine is 100 percent effective in children ages to 12 to 15, according to recent clinical trial data. Currently, the vaccine is only authorized for use so far on an emergency basis for people over the age of 16.

Overall, how long immunity lasts after receiving the Pfizer vaccine remains unclear. As mentioned earlier, current science estimates coronavirus immunity only lasts 3 to 12 months. But that doesn't mean the vaccine won't be more durable than that estimate. Still, booster shots may become a normal public health measure in the next few years.

"The information coming from Pfizer-BioNTech is good news with evidence that those enrolled in the clinical trials last year are still protected. So we know that immunity will not be short-lived," Dr. Peter Hotez, a vaccine expert and dean of the school of tropical medicine at Baylor College of Medicine, told CNN. "Hopefully the protection might last years, but we won't know until we know."

By Nicole Karlis

Nicole Karlis is a senior writer at Salon, specializing in health and science. Tweet her @nicolekarlis.

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Covid-19 Immunity News Brief Pandemic Pfizer Vaccines