Scientists in the U.S. say that research suggests that transfusions of blood products might help to cut deaths in the event of a bird flu pandemic.

They have arrived at this conclusion following an examination of records from the Spanish flu pandemic of 1918-1920 which killed up to 100 million worldwide.

The Spanish flu outbreak has particular relevance because the virus behind it is a close relative of the H5N1 virus.

When they looked at the records in detail they found transfusions taken from people who had recovered may have improved the condition of others hospitalised by the virus.

This latest research was a joint venture by various groups including the U.S. Navy, and suggests that blood transfusions might be an effective addition to the treatment options of vaccines and anti-viral drugs, in treating bird flu.

Despite the H5N1 strain of bird flu currently doing the rounds being a disease in the main of animals, which is quite hard for humans to contract, experts continue to worry that the virus could mutate and acquire the ability to pass easily from human to human.

This could threaten millions of lives.

To date 140 in Asia have died from the virus and millions of birds have been either killed by it or culled because of it.

The World Health Organization (WHO) has already warned that the world is not prepared for a flu pandemic, and a vaccine will take time to prepare and distribute following the first outbreaks.

The WHO also warns that most developing countries will have no access to vaccines and antiviral drugs throughout a pandemic and anti-viral drugs elsewhere might also be initially in short supply.

Current recommended treatment is hospitalization and treatment with an antiviral drug, but studies of bird flu outbreaks in Asia have found that 30 percent to 80 percent of patients treated this way have died.

According to the researchers just one recovering patient could donate enough blood plasma to treat many others.

They do admit that the records on which they based their study were limited as the number of patients was small, the work was not scientifically well controlled, and dosages were not standardised.

As a result of their findings the researchers are calling for experts to examine the potential for plasma therapy, which is also known as serotherapy, in greater detail, and to consider the benefits of drawing up treatment guidelines.

Other experts say a similar approach has been used to treat other viral diseases but some cast doubt on the theory and say it poses logistical problems of how to obtain, classify and prepare blood materials in the midst of an outbreak.

They also warn that any treatment involving blood products carries the potential for the transmission of other infections and they maintain that vaccines and anti-virals were more probable front line defences, along with basic public health measures, such as hand washing and wearing masks.

The WHO says the H5N1 virus is widespread in birds, and endemic in poultry in parts of Asia which is the source of the current infections.

The research is published in Annals of Internal Medicine.