JOHANNESBURG, Dec 16 (Reuters) - A small study of seven COVID-19 cases in South Africa shows the Omicron variant can break through booster vaccine shots, one of the scientists involved said on Thursday. A group of seven German visitors to Cape Town, aged between 25 and 39, were infected at some point in late November or early December with confirmed cases of the Omicron coronavirus variant. A copy of the study seen by Reuters said they all suffered only mild to moderate symptoms and none required hospitalisation. Of the group, six were fully vaccinated with the Pfizer-BioNTech (PFE.N) vaccine, of whom five had also had a booster Pfizer shot while the sixth had received a booster dose of the Moderna (MRNA.O) vaccine. A seventh had received an initial dose of AstraZeneca"s vaccine (AZN.L), followed by a second and then third booster dose of Pfizer. None had reported past infection with COVID-19. The infections occurred between one and two months after receipt of the booster shot. Wolfgang Preiser, a virologist at Stellenbosch University and one of the study"s co-authors, told Reuters the study was currently under peer review. He said the study showed infection was possible and caused symptomatic disease even after boosted immunisation. A much bigger real world study of 581 people in Britain showed booster vaccines significantly restore protection against mild disease caused by the Omicron variant, in part reversing an otherwise steep drop in vaccine effectiveness, the UK Health Security Agency said last week. read more Scientists suspect that Omicron is more transmissible, given its rapid spread, but say it is too early for definitive conclusions on the severity of the disease it causes. read more Medical professionals say vaccination is still an essential tool in the fight against the pandemic. "It still protects against severe disease and death: we are quite sure about that," Preiser said. However, he added, "the picture emerging is that this virus is very capable of undermining the immune response ... (so) it"s clear that we will need an updated vaccine."