Interim results published on Friday said that a single dose of Johnson & Johnson’s experimental Covid-19 vaccine produced a strong immune response against novel coronavirus in early-to-mid stage clinical trial. The vaccine is called Ad26.COV2.S. It was well tolerated at two different doses, as per the results. A single shot of the vaccine could simplify the distribution of the vaccine.

It is still unclear whether elderly people will be protected to the same degree as younger people with Johnson and Johnson vaccine.

The results, released on the medical website medRxiv, have not been peer-reviewed.

In July, the vaccine was found to offer strong protection in a single dose to monkeys. The vaccine is backed by the US government. The trial in about 1000 healthy adults started after the vaccine elicited a strong immune response.

On Wednesday, Johnson & Johnson kicked off a final 60,000 person trial. This may pave way for regulatory approval post an application.

Researchers, including those from J&J’s unit Janssen Pharmaceuticals, said 98% of participants with data available for the interim analysis had neutralizing antibodies, which defend cells from pathogens, 29 days after vaccination.

However, immune response results were available from only a small number of people – 15 participants – over 65 years old, limiting the interpretation.

In participants older than 65, the rate of adverse reactions such as fatigue and muscle aches was 36%, much lower than the 64% seen in younger participants, the results showed, suggesting the immune response in older people may not be as strong.

The researchers said more details on safety and effectiveness will follow when the study is completed.

For now, the results justify why more studies are needed in larger numbers to look for serious adverse effects, Dr. Barry Bloom, a professor at Harvard T.H. Chan School of Public Health who was not involved in the J&J trial, told Reuters.

“Overall, the vaccine is doing what you would expect it to do if you were to move it to Phase 3 trials,” Bloom said.