GlaxoSmithKline said it would move its experimental vaccine against the respiratory syncytial virus (RSV), a cause of pneumonia in toddlers and the elderly, into the final stage of testing, encouraged by mid-stage trial results.

GSK said a Phase I/II trial with about 1,000 healthy adults aged 60-80 showed that the vaccine prompted a “robust” increase in antibodies and immune cells one month after injection, indicating a stimulated immune system.

A separate product, designed to give pregnant women the ability to confer immunity to their unborn children, led to high levels of protective neutralising antibodies in non-pregnant healthy woman taking part in Phase I/II trial.

GSK said Phase III studies, with the potential to produce data for regulatory approval, would likely start over the coming months.

The vaccines were well-tolerated with side effects including injection-site pain and headache, GSK added.

Companies including J&J, Sanofi and Moderna are competing to get a vaccine approved against RSV, which globally leads to about three million hospital stays for children under five per year.

Swedish Orphan Biovitrum’s Synagis, a monthly shot, is currently the only preventive treatment against RSV in high-risk infants. Sanofi and partner AstraZeneca are working on longer-acting nirsevimab, which could be given once per cold season to that group, if approved.

GSK is also conducting early-stage studies of an RSV vaccine for children and plans to publish results soon for a subgroup of children which already had some exposure to the virus.