Doctors fear that 88 patients at St George's Hospital in Tooting could be at risk after routine checks found equipment used to treat them might not have been sterilised properly.
The problem is with the endoscopes - equipment used to investigate symptoms within the gastric tract. The hospital said that patients who visited the endoscopy unit between 13 and 19 August "may have been at a slightly increased risk of cross infection". All those patients are being contacted and offered a vaccination against the virus.
Hepatitis B, which infects up to a third of the world's population, is usually transmitted through sexual intercourse, sharing needles or badly sterilised medical instruments. Occasionally an acute infection can be severe, with abdominal pain and jaundice. In the most serious cases patients may develop cirrhosis or liver cancer.
The World Health Organisation estimates that around the world 350 million people are chronically infected. The UK, says the Department of Health, has one of the lowest rates of infection at 1 in 1,000.
A spokesman for St George's said: "All patients are being contacted personally. We have no reason to believe that any of the patients were hepatitis positive. However ... it is our responsibility to be sure that all our patients are offered protective vaccination."
He said so far 83 of the 88 patients had been contacted and every effort was being made to reach the remaining five.
Lesley Stuart, chief officer of Wandsworth Community Health Council, said that, although she was concerned about the checks and maintenance of medical equipment, "we are reassured by St George's that they have amended their protocol ... and contacted the manufacturers, suggesting a modification."Reuse content