TWO patients have died in hospital after contracting Norovirus in the second deadly outbreak of the winter vomiting bug to strike Scotland in less than a week.

Norovirus was a "contributory factor" in the deaths of both patients at Perth Royal Infirmary. A spokeswoman for NHS Tayside said the individuals also had underlying health conditions.

The deaths come in the wake of another deadly outbreak in the NHS Greater Glasgow and Clyde area.

Six patients at the Royal Alexandra Hospital in Paisley, Renfrewshire, have died in the past week after contracting the bug, although norovirus was said to have been a "contributory factor" in only one of the deaths. All six patients had serious underlying health complaints.

A spokeswoman for NHS Tayside said: "There have been two deaths in NHS Tayside Hospitals where patients have died due to underlying health conditions and norovirus was recorded on their death certificates as a contributory factor."

Two wards at Perth Royal Infirmary remain closed after several patients fell ill with vomiting and diarrhoea, believed to be caused by norovirus infection. The outbreak was first reported there last week.

Statistics for norovirus during the winter of 2010/11 – a four-month period running from December through to the end of March, when the bug is expected to be at its most prevalent and lethal – showed there were just 10 cases where norovirus contributed to someone's death.

During the previous three winters there were 18 in 2009/10, three in 2008/09 and two deaths in 2007/2008.

Despite the deaths of three people during what is supposed to be a downtime for the bug, Professor Hugh Pennington, of Aberdeen University, one of the UK's foremost bacteriologists, said the recent spate of deaths were not a cause for alarm.

He said: "The season, in terms of high incidence, is going on longer than you would expect.

"My feeling is we're seeing these tragic cases because the norovirus is very busy at the moment. It's not a fault of the hospitals, it's just an indicator of how busy the virus is. It's been late in coming but it's come with a vengeance."

Mr Pennington said it was impossible to tell whether the outbreaks in Perth and Paisley were related to the same strain.

He said: "We don't have a lab that can do all that stuff, although I'm sure people will be looking at these strains of the virus to see if there's anything unusual about them.

"Norovirus mutates all the time and that's obviously a question we need to get an answer to. For example, is the virus common now just because a new strain has emerged?

"It doesn't change the way we fight the virus. But it would be important to know from a research point of view."

Yesterday the outbreak at the Royal Alexandra appeared to be easing, with the number of patients being treated for sickness and diarrhoea down to 11 compared to 22 on Wednesday.

However, seven wards remain closed to new admissions.

Meanwhile, speaking at First Minister's Questions, Scottish Labour's Shadow Health Secretary Jackie Baillie added to calls for the creation of a specialist laboratory to look at the dangers of the norovirus infection.

The Scottish Government said its Healthcare-Associated Infections Task Force had already explored the possibility but concluded there was no "public health benefit at this time".