Merck's pneumococcal vaccine showed strong immune responses in children and teens who are at higher risk of serious illness in a late-stage study, the drugmaker said on Thursday.

The shot, Capvaxive, triggered immune responses against 21 strains of the bacteria responsible for pneumococcal infections, which can cause severe illnesses including pneumonia, meningitis and sepsis.

Merck was testing the shot against its older vaccine PPSV23 in children aged two years and above and teens aged below 18 years, who have completed a primary pediatric pneumococcal vaccination regimen, and have one or more chronic medical conditions that put them at increased risk.

Capvaxive is already approved for adults in the U.S., European Union, Japan and other countries.