French authorities have opened a formal investigation into two managers from drugs groups GlaxoSmithKline and Sanofi Pasteur over a vaccination campaign in the 1990s, a judicial source said late on Thursday.

Judge Marie-Odile Bertella-Geffroy also opened an investigation for manslaughter against Sanofi Pasteur MSD, a joint venture between Sanofi Aventis and Merck , the same source said.

The investigations follow allegations that the companies failed to fully disclose side effects from an anti-hepatitis B drug used in a vaccination campaign between 1994 and 1998.

There was no immediate comment from the companies or the two managers involved.

From 1994 to 1998, almost two thirds of the French population and almost all newborn babies were vaccinated against hepatitis B, but the campaign was suspended after concerns arose about possible secondary effects from the treatments.

Some 30 plaintiffs have launched a civil action in the case, including the families of five people who died after vaccination.

(Reporting by Thierry Leveque; Writing by James Mackenzie; Editing by David Holmes)