Media reports on new data regarding AstraZeneca's experimental Covid-19 vaccine sparked confusion Wednesday about whether the vaccine candidate can reduce transmission of the novel coronavirus. Here's what the data found on the experimental vaccine's efficacy, potential effect on virus transmission, and more.

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What does the data show on efficacy?

AstraZeneca on Wednesday released a preliminary analysis on its Phase III clinical trials—conducted by AstraZeneca and Oxford University, which helped develop the experimental vaccine—on the vaccine candidate that took place in Brazil, South Africa, and the United Kingdom. The efficacy results included in the analysis—which was published as a preprint study in The Lancet—were based on data collected from a total of 17,177 trial participants in those countries.

Overall, AstraZeneca said the data showed that its experimental Covid-19 vaccine is safe and effective at preventing Covid-19.

According to AstraZeneca, the data showed that the vaccine candidate was 100% effective at preventing severe cases of Covid-19 and hospitalizations from the disease. The company said there were no reported cases of severe Covid-19 or hospitalizations from the disease among participants who received the vaccine candidate at more than 22 days after their first dose. In comparison, there were 15 reported hospitalizations from Covid-19 among participants in the control group, who received a placebo.

In addition, AstraZeneca said the data showed the vaccine was about 76% accurate at preventing Covid-19 overall after participants received their first dose of the two-dose vaccine candidate, with that protection lasting until the second dose. Among participants who received their second dose, the vaccine's efficacy increased to about 82% at an interval of 12 or more weeks, AstraZeneca said. In total, 332 of the trial's participants became infected with symptomatic cases of Covid-19 more than 14 days after receiving their second dose of the vaccine candidate, the data showed.

Mene Pangalos, EVP of BioPharmaceuticals R&D at AstraZeneca, said the "analysis reconfirms that our vaccine prevents severe disease and keeps people out of hospital."

Further, Pangalos said the analysis suggests it might be advantageous for patients to receive their second dose of AstraZeneca's vaccine candidate at a longer interval than other two-dose Covid-19 vaccines, which could help accelerate efforts to get more people a first dose. "[E]xtending the dosing interval not only boosts the vaccine's efficacy, but also enables more people to be vaccinated upfront," Pangalos said.

What does the data show on coronavirus transmission?

AstraZeneca said the data also showed that its Covid-19 vaccine candidate may reduce asymptomatic transmission of the novel coronavirus, based on results collected from weekly coronavirus testing among trial participants in the United Kingdom.

"The data showed that [polymerase chain reaction test] positive readings were reduced by 67% … after a single dose, and 50% … after the two-dose regimen, supporting a substantial impact on transmission of the virus," the company said. In the preliminary analysis, the study's authors wrote, "[This] data indicate[s] that [the AstraZeneca vaccine], used in the authorized schedules, may have a substantial impact on transmission by reducing the number of infected individuals in the population."

Some observers hailed the data as evidence that the vaccine candidate reduces transmission of the novel coronavirus. For instance, Matt Hancock, the British health secretary, on Wednesday said, "We now know that the … vaccine also reduces transmission and that will help us all get out of this pandemic." He added the data "should give everyone confidence that this jab works not only to keep you safe but to keep you from passing on the virus to others."

But some experts said those claims aren't accurate. That's because although the data indicated that "fewer people were carrying the virus as a result of being vaccinated," outside experts said using that data to suggest "proof of decreased transmission from people who are vaccinated" is "a big and unjustified leap," STAT News reports.

Carlos del Rio, a professor of infectious diseases at the Emory University School of Medicine, told STAT News that "[t]he study showed a decrease in [viral] shedding, not 'transmission.'" He added, "The bottom line is, no, one cannot draw a conclusion or straight line."

Kathleen Neuzil, director of the Center for Vaccine Development at the University of Maryland School of Medicine, had a similar take. "There are too many, in my view, moving variables to make too much of one single result here," she told STAT News.

But Neuzil also noted that the results are promising when it comes to vaccines' potential effects on transmission and in line with data on other Covid-19 vaccines. "The trend … is consistent and in the right direction," she said.

Natalie Dean, a biostatistician focused on vaccines at the University of Florida, however, said the analysis' results around viral shedding were confusing, as they suggested that one dose of the vaccine reduced positive coronavirus test results by a greater percentage than two doses of the vaccine, with a reduction of about 67% after the first dose compared with about 50% after two doses.

"Biologically how do we explain that? Does that signal that those numbers cannot be directly compared?" Dean said. "You just have certain expectations and that one is hard to understand. How would efficacy go down after receiving a booster?" she asked.

A need for more research

Ultimately, del Rio said AstraZeneca needs to conduct more research on its experimental vaccine in order to truly understand its effects.

Doug Brown, CEO of the British Society for Immunology, had a similar response. Brown said while evidence showing a Covid-19 vaccine reduces coronavirus transmission "would be extremely welcome news, we do need more data before this can be confirmed and so it's important that we all still continue to follow social distancing guidance after we have been vaccinated."

According to STAT News, AstraZeneca currently is conducting a large study of the vaccine candidate in the United States, and it may have results from the trial as soon as this month. The company could have enough data to seek an emergency use authorization from FDA in early March, the New York Times reports.

Anna Durbin, a vaccine researcher at Johns Hopkins Bloomberg School of Public Health who is working on the U.S. trial, told STAT News, "I just keep telling my volunteers that the U.S. study is the one that's going to be the definitive study to tell us how this vaccine works against severe, hospitalized Covid and symptomatic Covid."

America continues vaccine rollout as new coronavirus cases, hospitalizations, and deaths level off

AstraZeneca's new vaccine data comes as the United States looks to ramp up its Covid-19 vaccination effort, and as the country continues to grapple with leveling—but still high rates—of new coronavirus cases, hospitalizations, and deaths.

CDC data shows that, as of Wednesday morning, the federal government had distributed about 55.9 million doses of the United States' two authorized Covid-19 vaccines, which are manufactured by Pfizer and BioNTech, as well as Moderna. Each of the vaccines require that patients receive two doses of the inoculations a few weeks apart.

According to CDC's data, a total of about 33.9 million doses of the vaccines had been administered to Americans as of Wednesday morning. Of those, about 27.2 million Americans had received "one or more doses," and about 6.4 million had received two doses, the data shows.

Meanwhile, data shows that daily rates of newly reported coronavirus cases, hospitalizations and deaths is the United States are leveling off, though they remain high.

According to data compiled by the Times, U.S. officials on Wednesday reported about 119,014 new cases of the novel coronavirus. As of Thursday morning, officials had reported a total of about 26.5 million case of the virus since America's epidemic began.

According to the Times, the United States' average daily number of newly reported coronavirus cases over the past week was 136,442—which is down by 30% when compared with the average from two weeks ago, when the United States was in the midst its worst peak yet in newly reported cases.

As of Thursday morning, data from the Times showed that the rates of newly reported coronavirus cases were "staying high" in Alabama, Montana, and Vermont, which have each reported a daily average of at least 15 newly reported cases per 100,000 people over the past week. In contrast, rates of newly reported coronavirus cases were "staying low" as of Thursday morning in Guam, Hawaii, North Dakota, Puerto Rico, and the U.S. Virgin Islands.

All other states and Washington, D.C., had been seeing comparatively higher rates of coronavirus transmission, but the daily average of newly reported coronavirus cases over the past seven days in those areas was "going down" as of Thursday morning, according to the Times' data.

U.S. hospitalizations for Covid-19, the disease caused by the novel coronavirus, remained high as of Wednesday, though the number of Americans hospitalized with Covid-19 has decreased significantly from highs seen last month. According to data from The Atlantic's COVID Tracking Project, there were 91,440 Americans with Covid-19 hospitalized for treatment on Wednesday, including 18,147 who were receiving care in an ICU and 5,920 who were on a ventilator.

The United States' rate of newly reported deaths linked to the novel coronavirus also has declined over the past two weeks, though it too remains high. According to data from the Times, U.S. officials reported about 3,843 new deaths linked to the virus on Wednesday. As of Thursday morning, officials had reported a total of about 450,689 U.S. deaths linked to the virus since the country's epidemic began.

(Herper/Branswell, STAT News, 2/3; AstraZeneca release, 2/3; Voysey et al., The Lancet, 2/1; New York Times, 2/4; Adam et al., Washington Post, 2/3; CDC data, 2/3; New York Times, 2/4; "The COVID Tracking Project," The Atlantic, accessed 2/4).