LONDON A rise in the number of English cases of the H1N1 influenza could indicate the start of a second wave of infections and two cases have shown resistance to the antiviral drug Tamiflu, health officials said on Thursday.
There were about 5,000 new cases of the virus, known as swine flu, reversing recent declines with an increase in the number of people being admitted to hospital.
Scotland also reported "a significant increase" in the number of people contacting their doctors with flu symptoms, the Scottish government said.
"We don't know whether this is the start of the next big wave that we were expecting this autumn but it is certainly something that's giving us concern," Chief Medical Officer Liam Donaldson said.
"It will probably be a week or two before we see whether this increase is sustained."
Last week officials had said Britain was "tantalisingly close" to fending off the flu as the number of weekly infections fell to about 3,000 from a peak at the end of July when more than 100,000 cases a week were being reported.
Roche Holding said last Friday there had been 23 reported cases in which the flu had been resistant to the antiviral drug Tamiflu.
Donaldson said the Health Protection Agency (HPA) had found two more cases in Britain taking the overall total to 25.
"The positive side of it is that so far these have not been strains that have then gone on and affected other patients, they have stayed with the patient in which they were isolated," Donaldson said.
"What would worry us is if we got a resistant strain that then started infecting people like the rest of the cases of flu that have occurred."
Flu viruses are prone to mutation and experts are not surprised that they would evolve towards resistance, just as bacteria develop resistance to antibiotics.
Most of those who have contracted the virus have suffered mild symptoms. However, 67 people in England with the flu have died, the HPA said.
Earlier this month, Britain lowered the potential number of deaths from swine flu in the worst case scenario to 19,000 from 65,000.
(Reporting by Michael Holden)