Taking into account the effectiveness demonstrated so far by experimental vaccines designed to help smokers quit, the team concluded the drugs would need to be cheaper, require fewer doses and get better results for widespread preventive vaccination of teens to be worthwhile.

"On the data that is currently available on NicVax and other nicotine vaccines these scenarios are unrealistic," said Coral Gartner, a researcher at the University of Queensland who led the study.

No anti-smoking vaccines are on the market yet, but several have been tested in humans. NicVax, being developed by Nabi Pharmaceuticals, is one for which trial results have been published -- but it didn't appear to help people quit smoking.

There's considerable interest in developing vaccines to reduce smoking rates, Gartner says. And there's been some discussion of the potential for using them to prevent youngsters from becoming smokers in the first place.

To see how realistic that idea is, Gartner and her colleagues used the results from a phase 2 clinical trial of NicVax to model how well it might actually work to curb smoking rates among teens.

Under the best case scenario, they found, the vaccine would decrease the number of Australian adolescents who start smoking between the ages of 12 and 19 from 10 percent per year to two percent.

"Since we finished our study, Phase III trial results (of NicVax) have become available that fail to show any benefit of a vaccine for smoking cessation, suggesting this 'best case scenario' is highly unrealistic," Gartner told Reuters Health by email.

Even if it did work, the cost is above the typical thresholds, at least in Australia, to consider a prophylactic vaccine worth paying for.

One of the big expenses that Gartner's team assumed in their calculations is that the shots would be given six times a year.

Given that requirement, each year of sickness or disability that the teens would avoid by not smoking would cost anywhere from about $80,000 to $530,000 -- well above the usual cut-off of $50,000.

"The findings from this study make sense for Australia, where strict cutoffs tend to be used with regard to funding vaccination programs," said Dr. Grace Lee, a professor at Children's Hospital Boston who did not participate in this research.

"Cost-effectiveness is one consideration, but not the sole consideration, with regard to decisions about vaccine use in the U.S.," she wrote in an email to Reuters Health.

For instance, a vaccine for adolescents that protects them from meningitis, and which is recommended for children in the U.S., is thought to cost more than $100,000 for each year of life saved by the shot, Lee pointed out.

Even though there are no nicotine vaccines available, she said, it's useful to determine their costs compared to their benefits.

"This might help either target the vaccine to a higher risk population, where the intervention might be considered cost-effective, or help vaccine developers focus on certain characteristics of the vaccine that would increase the likelihood of adoption by a national vaccination program," she said.

In other countries that have higher smoking rates, a nicotine vaccine program might be more cost effective.

Gartner said that, based on her results, a smoking vaccination program for teens is unlikely to receive public funding and become a reality in Australia.

"Rather than seeking technological fixes like preventive vaccination, we should focus on strategies that have greater potential to discourage smoking uptake and encourage quitting," she said.

These include banning tobacco advertising, requiring plain packaging on cigarette boxes, and having high taxes on tobacco.

SOURCE: bit.ly/ycKsln Addiction, online January 23, 2012.

• A Permanent Economic Emergency