MONTREAL (CP) - Wider use of chickenpox vaccine could produce two side-effects, one a blessing and the other a curse, infectious disease experts said Monday.

On the positive side, there would likely be fewer cases of flesh-eating disease in young children. On the negative side, there could be higher rates of shingles among adults.

Experts gathered at an immunization conference in Montreal were enthusiastic about the varicella vaccine. In addition to eliminating the misery of itchy chickenpox, wide use of the vaccine would cut the 300 to 350 hospitalizations a year the disease provokes across Canada, as well as the risk of a child developing life-threatening necrotizing fasciitis, they said.

A study presented at the conference showed that between September 2001 and August 2003, 37 children in Canada had flesh-eating disease and two of them died. In 61 per cent of the cases, the children had chickenpox - a known risk factor - in the month before developing necrotizing fasciitis.

Most of the affected children were under one year old, meaning they were too young to have been vaccinated against varicella, the scientific name for chickenpox, said Dr. Danielle Grenier, medical affairs officer for the Canadian Pediatric Society, one of the co-sponsors of the conference.

But if enough children over one year old are vaccinated, the amount of circulating virus - and the risk to younger children - will drop dramatically, a phenomenon known as herd immunity.

"We could prevent not only necrotizing fasciitis, but also prevent the other complications of varicella," Grenier said.

"But if we get herd immunity, it would also protect the little ones, the ones that were most severely affected by this."

Currently only Alberta, Ontario, New Brunswick, Nova Scotia, Prince Edward Island, Northwest Territories and Nunavut provide varicella vaccine for free as part of their pediatric vaccination programs.

But increased use of the vaccine in children could come at a cost to adults who had chickenpox earlier in their lives.

The varicella virus doesn't leave one's system, but rests in a latent state in nerve cells. For some people, it stays that way. But in others, the virus becomes reactivated later in life, causing a painful condition called herpes zoster - more commonly known as shingles.

It's believed that adults who have intermittent exposure to wild chickenpox virus get what amounts to a natural booster shot.

If widespread use of varicella vaccine dramatically reduces the amount of circulating wild virus, immunity to the virus could wane in adults and more people could have flare-ups of shingles, it is believed.

"At this point virtually every Canadian adult has had real chickenpox, so a quarter of us will be expected to get shingles at some point. So that may go up without wild virus circulating," said Dr. David Scheifele, director of the vaccine evaluation centre at B.C. Children's Hospital.

The condition isn't just painful, it is costly.

Data presented at the conference by pharmaceutical manufacturer Merck Frost, which markets a varicella vaccine, suggest there are about 130,000 cases of shingles a year in Canada and they cost the health care system somewhere between $39 million and $82 million a year.

Dr. Barb Law insisted it isn't clear how much protection adults get from exposure to wild virus versus how much comes from their own immune systems. Still, Law - an infectious disease expert from the University of Manitoba and a leading proponent of the varicella vaccine - acknowledged the risk of a 20 per cent rise in shingles cases exists.

But she doesn't believe it should get in the way of the pediatric immunization program.

"To me, it's absolutely not a reason not to start immunizing, because there's a burden with chickenpox with children."

And the risk may be lowered, Law and Scheifele noted, by the introduction in the not-too-distant future of an adult chickenpox booster shot.

Vaccine manufacturers have been studying the efficacy of giving adults a varicella booster as a means of preventing shingles. Findings are expected to be published sometime next year.