Johnson & Johnson paused dosing and enrollment in all of its COVID-19 vaccine clinical trials due to an unexplained illness in a study participant, the company announced Monday.
J&J said on Monday the illness was being reviewed by an independent data and safety monitoring board as well as the USA group's clinical and safety physicians.
"Even if we had enough vaccine for everyone, it's unlikely that ... the physical distancing rules can be just dropped", Prof Andrew Pollard told the Daily Mail.
J&J's trial is on a study pause as opposed to a regulatory hold - a regulatory hold of a clinical trial is a requirement by a regulatory health authority, such as the US Food and Drug Administration.
Pfizer Inc PFE.N has said it will enroll children, who are capable of passing on the virus to high-risk groups, as young as 12 in its large, late-stage COVID-19 vaccine trial, while AstraZeneca AZN.L has said a sub-group of patients in a large trial will test children between five to 12.
The company, which reports quarterly financial results on Tuesday morning, said such pauses are normal in big trials, which can include tens of thousands of people. AstraZeneca has since resumed trials in all countries except the US, in which it is working with the FDA to ensure all necessary precautions are taken to proceed with safety.
Like the AstraZeneca vaccine candidate, Johnson & Johnson's vaccine relies on an experimental method, based on an adenovirus vector, that has yet to lead to any approved immunizations, although it was used previously in studies to prevent infections of influenza and Ebola. Other vaccine candidates in the US require two shots.
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"This could be a second case of adenoviral vaccine to spur safety concerns", said Bryan Garnier analyst Olga Smolentseva.
The coronavirus vaccine research will test how protects a range of people.
"This is likely to be a neurological event", he said. Last month, J&J said its vaccine candidate produced a strong immune response in an early-to-mid stage clinical trial.
The J&J vaccine candidate dosed its first volunteer participant on September 23.
It is not known-and "is not always immediately apparent", J&J added-if the ill participant was among participants randomized to JNJ-78436735, or to placebo.
The pause allows the company to determine whether the illness is related to the COVID-19 vaccine and whether to resume the study.