The results, published in the October issue of Vaccine, follow earlier studies that also raised questions about the vaccine's effectiveness.

"The protective effect of Pneumovax II vaccination on specific antigens in high-risk individuals in vivo will at maximum only be around 30%," they write.

Pneumovax II (Aventis Pasteur MSD) is used in high-risk patients in whom pneumococcal infections could be life threatening. These include the elderly, those who have had their spleen removed or suffer from diabetes, and patients with chronic diseases of the heart, kidneys, lungs or liver.

After previous research raised doubts about its effectiveness, the St. George's Hospital Medical School team tested the vaccine in the lab to see how effective it was in bolstering immunity.

The researchers reasoned that if the vaccine was to be effective it needed to at least double concentrations of specific antibodies to pneumococcal infection. But it also needed to increase serum opsonic capacity--a measure of capacity to enhance phagocytosis of the pneumococcal bacteria. In phagocytosis, immune system cells "swallow" the infectious agent.

The team recruited the 16 male and female patients and measured their level of natural immunity acquired from exposure to infection in the past.

They then measured immune response to the pneumococcal S14 antigen, which is one of the components of the Pneumovax II vaccine.

The results showed that only six out of the 16 volunteers experienced an increase of 100% or more in antibodies to S14. Separate tests revealed only five of them also had an increase in opsonic capacity.

Huo and colleagues acknowledged the study was small but added: "The results add useful laboratory information to the controversial effect of Pneumovax II vaccination in high-risk individuals."

Dr. Jonathan Van-Tam, medical director of Aventis Pasteur MSD, told Reuters Health the findings had to be considered in the context of much bigger epidemiological studies that showed it was effective in preventing infection.

"This was a small study and looked at the activity of one strain which is known to show resistance. The whole point about the vaccine is that it's formulated with 23 different serotypes and covers 90% of the organisms likely to be involved."

"Other studies have shown the vaccine is about 65% effective."'