Three doses of a Covid-19 vaccine provide stronger protection against the highly contagious omicron variant than two primary series doses, according to a new Danish study, a finding that provides more support for the effectiveness of boosters as the U.S. government prepares to roll out shots specifically targeting the omicron strain.
Primary series Covid vaccines provided weak protection against omicron infections, according to the study, which was published in PLOS Medicine: Among those aged 12 to 59, two doses were only 40% effective in preventing omicron infections 14 to 30 days after vaccination, which fell to 12.6% four months after inoculation.
Three Pfizer-BioNTech or Moderna vaccine doses, meanwhile, provided 55.1% protection against omicron infection among those ages 18 to 59, which fell to 52.3% after four months, with similar rates observed in those 60 years and older, according to researchers, who examined data on Denmark residents 12 years and older.
A booster shot also appeared to provide stronger protection against omicron hospitalizations: Vaccines were 83.3% effective at preventing hospitalizations four months after a third shot among those 60 and older, compared to 77.6% protection four months after two doses among those ages 18 to 59 (those 60 and older with only two shots weren’t included, as nearly all had opted for a third dose by the omicron wave).
The study indicates a third dose “is necessary to maintain protection” against coronavirus infections for a longer time and to guard against hospitalizations in light of the new omicron variant, the authors wrote.
The Biden administration could roll out new Covid booster shots targeting omicron subvariants BA.4 and BA.5 as soon as Labor Day weekend. The Food and Drug Administration authorized Pfizer and Moderna’s updated shots this week, which, after approval from the Centers for Disease Control and Prevention, will be available to all Americans 12 years and older two months after their last shot.
Previous research has found only primary series vaccination or a previous Covid infection provide little protection against symptomatic omicron infections, while other studies have demonstrated booster doses provide extra protection against hospitalizations, emergency room and urgent care visits. The omicron variant, first reported in South Africa in November 2021, fueled a surge in winter coronavirus infections in the U.S. that led to a spike in Covid hospitalizations and deaths. The variant has specific mutations in its spike proteins—the part of the virus that binds to the human cell—that helps it evade the immune system as well as antibodies from previous coronavirus infections and vaccines. The Biden administration is hoping to stave off a winter Covid surge with new shots targeting omicron substrains BA.4 and BA.5, which have quickly become dominant after making their way to the U.S. in late March.
By Madeline Halpert, Forbes Staff