US biopharmaceutical company Pfizer Inc (NYSE:PFE) and vaccine company Valneva SE (Nasdaq:VALN) (Euronext Paris:VLA) will discontinue testing of an experimental Lyme disease vaccine in around half of the people taking part in a US trial, the two companies said on Friday.

This follows violations of clinical trial guidelines at certain sites run by a third-party operator, Pfizer and Valneva explained.

They added that the discontinuation with these participants "was not due to any safety concerns with the investigational vaccine and was not prompted by a participant-reported adverse event".

The Phase 3 clinical study is investigating the efficacy, safety and immunogenicity of VLA15, an investigational Lyme disease vaccine candidate. It remains ongoing at other sites not operated by the third party, and Pfizer is continuing to recruit new participants at those sites.