Two Covid-19 vaccines stalled by potential side effects have one key feature in common: Both are based on adenoviruses, cold germs that researchers have used in experimental therapies for decades with varying results.
Johnson & Johnson said late Monday it would pause its trial to investigate an illness, which it didn’t specify, in a study participant. Meanwhile, AstraZeneca Plc’s U.S. trial of the vaccine it’s developing with the University of Oxford has been halted by regulators for more than a month after neurological symptoms arose in two volunteers.
With AstraZeneca in a pit stop, vaccines from Moderna Inc. and the Pfizer Inc.-BioNTech SE partnership have taken the lead in the race to be first out with a shot. Meanwhile, the two paused trials are reviving questions about adenoviral vectors, which have been used in laboratory, animal and human experiments for years. In some cases, the experiments have succeeded, but not always.
And this year, with Covid-19 vaccines entering strongly into the politics of the hour, transparency and trust are key to fighting a virus that’s hit more than 39 million people globally and hamstrung economies. If concerns about side effects in experimental vaccines in trials using adenoviruses are validated, it could boost skepticism in the general public and raise questions for other drugmakers.
“While it could be a coincidence,” said Sam Fazeli, a Bloomberg Intelligence analyst, in a research note, “there’s still the possibility that adenoviral vector vaccines run a higher risk of rare side effects – such as autoimmune attacks like transverse myelitis – than those of Pfizer-BioNTech, Moderna or Novavax.”
Pauses to investigate side effects aren’t unusual in vaccine trials, which require a high safety bar because they’re taken by healthy people. Oxford has said there’s insufficient evidence to connect the participants’ illnesses to their vaccine. Its human tests in the U.K., South Africa and Brazil resumed weeks ago.
Adenoviral vectors are well studied, versatile, and shown to be well tolerated, making them good candidates for Covid vaccines, AstraZeneca said in an email. Reactions to the Astra/Oxford vaccine in early studies were comparable to those seen in previous trials of other vaccines using adenoviruses, the company said. Oxford researchers declined to comment.
If adenoviruses are associated with the side effects that have appeared in Covid vaccines, it may set back development of numerous projects, as it did with HIV and gene therapy. There are more than a dozen Covid vaccines in development based on adenoviruses, according to the World Health Organization.
Should investigators in the current trials determine the cause of the episodes is related to the vaccines, they would look for potential links to the adenovirus approach as well as to the spike protein the vaccine is designed to make to prepare the immune system for a real infection, according to Washington University’s Kinch.
At this point, he said, there’s not enough information to know. “Is this just random chance?” Kinch said. “First and foremost, there’s bad luck. If it turns out there’s a correlation and a causation, then the conversation very quickly turns.”

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