Study Finds Pfizer And AstraZeneca Vaccines Effective Against The Delta Variant — As Long As You Get Both Doses

Published 2 years ago
A doctor’s hand with a syringe of covid-19 coronavirus vaccine.

TOPLINE Both the Pfizer and AstraZeneca coronavirus vaccines work well at preventing symptomatic cases of the delta coronavirus variant—which became the dominant strain in the U.S. this month—but the vaccines’ effectiveness drops significantly in patients who only have one of the two doses, according to a new study.


A study of 19,000 people from Public Health England published Wednesday found that both doses of either the Pfizer or AstraZeneca vaccines help protect against symptoms from both the alpha (B.1.1.7, also known as the U.K. variant) and delta variants at comparable rates.

Two doses of the Pfizer vaccine had an effectiveness of 93.7% at preventing symptomatic alpha infections, and 88% for the delta variant.


Two doses of AstraZeneca were found to be 74.5% effective at preventing symptomatic alpha infections, and 67% for delta.

However, the effectiveness of the vaccines plummeted if a patient had only received one dose of either vaccine: the study found both to be just more than 48% effective against alpha and only about 30% effective against delta.

Researchers said their findings support the theory that prioritizing second doses, especially in vulnerable populations, is an effective way to prevent coronavirus outbreaks. 


The single-dose Johnson & Johnson vaccine only has a 33% efficacy rate against the delta variant, a New York University study released Tuesday suggested, although it has not been peer reviewed. A small study from Moderna last month suggested their vaccine also offers protection against the delta strain.



The delta variant, which experts believe is more transmissible than earlier forms of coronavirus, is thought to be the driving force behind a new wave of infections and a jump in hospitalizations across the U.S. Experts have warned about the danger delta could pose. Former Food and Drug Administration head Dr. Scott Gottlieb said the “majority” of unvaccinated Americans would likely contract the delta mutation eventually, adding the disease “is going to be the most serious virus they will get in their lifetime in terms of the risk of putting them in the hospital.” The surge of new cases has pushed some cities like Los Angeles and Las Vegas to reintroduce mandatory masking in some indoor spaces.

By Carlie Porterfield, Forbes Staff