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March 25, 2021

AstraZeneca just released new data on its Covid-19 vaccine. Here's what it shows.

Daily Briefing

    AstraZeneca on Wednesday released updated clinical trial data on its Covid-19 vaccine candidate that showed the vaccine is 76% effective at preventing Covid-19 and highly effective at preventing severe cases and hospitalizations from the disease.

    Health officials had raised concerns about AstraZeneca's Covid-19 vaccine data

    The new analysis comes after an unusual few days in which the National Institute of Allergy and Infectious Diseases (NIAID) raised public concerns about AstraZeneca's previously released vaccine data.

    The back-and-forth began on Monday, when AstraZeneca issued a press release announcing preliminary data from a 32,449-participant clinical trial of its two-dose vaccine candidate. According to the release, the vaccine was found to be 79% effective in preventing symptomatic Covid-19.

    However, NIAID after midnight on Tuesday morning issued a statement raising questions about the data. NIAID said the Data Safety Monitoring Board (DSMB)—an independent panel of experts responsible for overseeing the trial under NIH—had notified AstraZeneca, the Biomedical Advanced Research and Development Authority, and NIAID that it was "concerned" by the data AstraZeneca had released.

    "The DSMB expressed concern that AstraZeneca may have included outdated information from that trial, which may have provided an incomplete view of the efficacy data," NIAID said.

    In response, AstraZeneca said the numbers "were based on a pre-specified interim analysis" and that it would "immediately engage with the independent [DSMB] to share our primary analysis with the most up to date efficacy data."

    New data offers similar estimates of Covid-19 vaccine's effectiveness

    In a press release Wednesday, AstraZeneca announced an updated analysis of its clinical trial data that showed largely similar results to the original analysis.

    According to the release, the new analysis was based on 190 symptomatic cases of Covid-19 among the trial's participants, which is 49 more cases than had been included in the earlier analysis. However, AstraZeneca said the reported number of cases among participants "may fluctuate slightly" because 14 possible or probable Covid-19 cases are currently being investigated.

    According to the release, the vaccine was 76% effective against symptomatic Covid-19 cases. AstraZeneca in the release noted that the statistical confidence interval for the vaccine's effectiveness ranged from 68% to 82%.

    In addition, AstraZeneca said no severe Covid-19 cases or hospitalizations had occurred among vaccinated trial participants. According to the release, eight cases of severe Covid-19 were observed in the trial population—with all of those occurring in the placebo group.

    AstraZeneca in the release stated that "[t]hese results have been presented to the independent [DSMB]." An AstraZeneca spokesperson did not further clarify the company's statement but noted that the release had been shared with the DSMB and the preliminary analysis was prespecified, STAT News reports.


    Based on the new data, some observers said it's unclear why NIAID issued a public statement concerning the initial data AstraZeneca had released on Monday.

    Eric Topol, director of the Scripps Research Translational Institute, said AstraZeneca's clinical trial results were "better than expected" given the public disagreement between the DSMB and AstraZeneca. Topol added that he's "relieved since the world needs the vaccine badly."

    Stephen Evans, a vaccines expert at the London School of Hygiene and Tropical Medicine, said, "Given the details given here, it seems an unnecessary action to have raised concerns in public. Results fluctuate as data accumulate ... what counts will be the FDA assessment and that will be done based on scrutiny of the full data and not press releases." He added that any vaccine with an efficacy rate higher than 60% is useful to the public.

    However, other observers said it remains unclear whether AstraZeneca's effectiveness estimate is higher than the DSMB's projections, the New York Times reports. Some researchers who spoke with STAT News said they would prefer to wait for further data to be released, including FDA's assessment of AstraZeneca's data, before they draw firm conclusions.

    For instance, Nahid Bhadelia, the director of the special pathogens unit at Boston Medical Center, wrote in a tweet, "At this point, I will wait for the FDA submission packet just to avoid any further roller coaster rides" (Weintraub, USA Today, 3/24; Robbins/Mueller, New York Times, 3/24; Neergaard, Associated Press, 3/25; Herper, STAT News, 3/24; AstraZeneca release, 3/25).

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