A spokeswoman for the Food and Drug Administration declined to comment.
A person familiar with the situation, who spoke on the condition of anonymity, said that the participant who experienced the suspected adverse reaction had been enrolled in a Phase 2/3 trial based in the United Kingdom. The individual also said that a volunteer in the U.K. trial had received a diagnosis of transverse myelitis, an inflammatory syndrome that affects the spinal cord and is often sparked by viral infections. However, the timing of this diagnosis, and whether it was directly linked to AstraZeneca’s vaccine, is still unknown.
Transverse myelitis can result from a number of causes that set off the body’s inflammatory responses, including viral infections, said Dr. Gabriella Garcia, a neurologist at Yale New Haven Hospital. But, she added, the condition is often treatable with steroids.
AstraZeneca declined to comment on the location of the participant and did not confirm the diagnosis of transverse myelitis. “The event is being investigated by an independent committee, and it is too early to conclude the specific diagnosis,” the company said.
Some said the company’s halt was evidence that the process was working as it should.
“At this stage, we don’t know if the events that triggered the hold are related to vaccination,” said Dr. Luciana Borio, who oversaw public health preparedness for the National Security Council under Mr. Trump and who was acting chief scientist at the F.D.A. under President Barack Obama. “But it is important for them to be thoroughly investigated.”
The Coronavirus Outbreak ›
Frequently Asked Questions
Updated September 4, 2020
What are the symptoms of coronavirus?
- In the beginning, the coronavirus seemed like it was primarily a respiratory illness — many patients had fever and chills, were weak and tired, and coughed a lot, though some people don’t show many symptoms at all. Those who seemed sickest had pneumonia or acute respiratory distress syndrome and received supplemental oxygen. By now, doctors have identified many more symptoms and syndromes. In April, the C.D.C. added to the list of early signs sore throat, fever, chills and muscle aches. Gastrointestinal upset, such as diarrhea and nausea, has also been observed. Another telltale sign of infection may be a sudden, profound diminution of one’s sense of smell and taste. Teenagers and young adults in some cases have developed painful red and purple lesions on their fingers and toes — nicknamed “Covid toe” — but few other serious symptoms.
Why is it safer to spend time together outside?
- Outdoor gatherings lower risk because wind disperses viral droplets, and sunlight can kill some of the virus. Open spaces prevent the virus from building up in concentrated amounts and being inhaled, which can happen when infected people exhale in a confined space for long stretches of time, said Dr. Julian W. Tang, a virologist at the University of Leicester.
Why does standing six feet away from others help?
- The coronavirus spreads primarily through droplets from your mouth and nose, especially when you cough or sneeze. The C.D.C., one of the organizations using that measure, bases its recommendation of six feet on the idea that most large droplets that people expel when they cough or sneeze will fall to the ground within six feet. But six feet has never been a magic number that guarantees complete protection. Sneezes, for instance, can launch droplets a lot farther than six feet, according to a recent study. It’s a rule of thumb: You should be safest standing six feet apart outside, especially when it’s windy. But keep a mask on at all times, even when you think you’re far enough apart.
I have antibodies. Am I now immune?
- As of right now, that seems likely, for at least several months. There have been frightening accounts of people suffering what seems to be a second bout of Covid-19. But experts say these patients may have a drawn-out course of infection, with the virus taking a slow toll weeks to months after initial exposure. People infected with the coronavirus typically produce immune molecules called antibodies, which are protective proteins made in response to an infection. These antibodies may last in the body only two to three months, which may seem worrisome, but that’s perfectly normal after an acute infection subsides, said Dr. Michael Mina, an immunologist at Harvard University. It may be possible to get the coronavirus again, but it’s highly unlikely that it would be possible in a short window of time from initial infection or make people sicker the second time.
What are my rights if I am worried about going back to work?
AstraZeneca’s vaccine uses a viral vector that ferries coronavirus genes into human cells. The viral vector in this case is a modified chimpanzee adenovirus, altered to render it harmless to people. The coronavirus components of the vaccine are intended to spark a protective immune response that would be roused again should the actual coronavirus try to infect a vaccinated individual.
In a paper published in The Lancet in July, researchers behind AstraZeneca’s formulation reported that the majority of participants in the vaccine’s Phase 1/2 trials, which are designed to assess the product’s safety, had experienced some mild or moderate side effects, including muscle aches and chills. None of the reactions, however, were considered severe or life-threatening, and resolved quickly. The vaccine was deemed safe enough to proceed to further testing.
AstraZeneca’s vaccine is in Phase 2/3 trials in England and India, and in Phase 3 trials in Brazil, South Africa and more than 60 sites in the United States. The company intended for its U.S. enrollment to reach 30,000, and started its American trials on Aug. 31.
Katherine J. Wu and Katie Thomas
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