THE number of patients being treated in intensive care after catching flu has trebled in a week as experts warn one of the key strains spreading around the country is not fully covered by the vaccine.

In their first bulletin of the year, health officials revealed there had been 10 reports of people falling seriously ill with flu complications in the last seven days.

This suddenly increased the total number of patients needing intensive care treatment due to flu in Scotland this winter to 15.

Rates of flu in England are already said to be the highest in four years and hospitals both sides of the Border have been struggling to cope with the number of sick patients - with a range of medical problems - who need beds.

Dr Jim McMenamin, epidemiologist for Health Protection Scotland and an expert on flu, said the number of consultations about flu symptoms with GP practices in Scotland are higher than last year, but below the levels of 2012-13 and within the normal range.

However, he added that there had been outbreaks in communities such as care homes where lots of people are covered by the annual flu vaccine. He explained there is a "potential mismatch" between one of the most common flu strains circulating in Scotland and the strain people are protected from by the jab.

He said, in common with North America and much of the rest of Europe, "we have seen the majority of influenza A H3N2 as being of a slightly different type compared to what we would expect. It is slightly different from what is in the vaccine."

Every year the World Health Organisation assesses the strains of flu which are most likely to hit the northern hemisphere during the coming winter and recommends what should be covered by the vaccine.

Protection from three types of flu are put into the injection, usually two forms of influenza A and one form of influenza B. Influenza A tends to cause more serious problems and is the virus most likely to mutate.

So far this winter specialist labs in the West of Scotland have genetically tested 50 samples of influenza A H3N2 from sick patients, finding 36 (76 per cent) were different from the H3N2 strain included in the vaccine. This issue, known as "antigenic drift" may mean the population is less well protected from flu this winter.

Dr McMenamin said the jab, which is still effective against two other types of flu, may offer enough "cross-immunity" to stop people getting H3N2 as well - but some vaccinated people have fallen ill. He continued: "Although it might not prevent everyone getting flu, our expectation is that their illness will not be as severe and it is less likely they will go to their GP or end up in hospital."

Based on testing so far, it would appear the variant strain is more dominant in Scotland than in England.

Dr McMenamin stressed people who are eligible for the flu vaccine should still take up the offer of the jab if they have not already had the injection. There is more than one strain of flu circulating, he said, and it is early in the season so the strain dominating may change. People over the age of 65, pregnant women, and people with long term conditions such as diabetes are among those who can have the injection on the NHS.

The first flu bulletin of 2015 covered the week ending Sunday January 4.