A new study has examined whether a vaccine that offers only partial protection can backfire by increasing risky behaviour, as a false sense of security could make recipients less careful than they otherwise would be.
While the study looks at the response to Lyme disease vaccination, its authors suggest the research also sheds light on the controversy over whether girls should receive the HPV vaccine to fight cervical cancer.
Ticks spread Lyme disease, which is usually treatable by antibiotics. However, late detection or a poor response to antibiotics can lead to lasting pain, fatigue and neurological problems. The vaccine, known is LYMErix, is about 80 percent effective.
Brewer and colleagues conducted a random phone survey of 705 adults, who lived in areas where Lyme disease is common, just after the vaccine became available. Eighteen months later, they re-interviewed participants and compared the behaviour of those who received the vaccination to those who had not.
The researchers looked for evidence of "risk compensation"— a phenomenon in which people maintain a relatively stable level of risk-taking. For example, some studies find that people "compensate" for risk reduction caused by seatbelt use by driving faster.
"When you talk about risk compensation, there are two things that could happen," Noel Brewer, Ph.D., assistant professor of public health at the University of North Carolina, lead author of the study said.
Vaccinated individuals might reduce other efforts to avoid Lyme; for example, they might stop wearing light-coloured clothing that makes ticks visible. If they reduced this behaviour only to the level of unvaccinated individuals, their risk would stay the same. Brewer calls this "regression."
Alternatively, people might become "disinhibited" by vaccination and less cautious than the unvaccinated, in which case vaccination might actually increase their risk because it is not 100 percent effective.
In the study, the authors found evidence of regression on two safety measures: wearing light-coloured clothing and using tick repellant. However, they found no evidence of disinhibition, which led Brewer to conclude that fears of disinhibition related to the HPV vaccine could be overblown as well.
"What on earth can Lyme disease vaccination tell us about HPV. Well, it sets you up to ask the right questions. In the place where risk compensation is most likely to happen — a disease where people know the risk and know how to prevent it — we find weak support for it," Brewer said.
On the other hand, with HPV, few people know that the sexually transmitted virus can cause cervical cancer; consequently, few take steps to prevent it because they are unaware of the risk. As a result, "there is no reason to expect to see disinhibition," Brewer said.
Baruch Fischhoff, professor of social and decision sciences at Carnegie Mellon University, is not convinced that the two vaccines are comparable.
"I have trouble extrapolating to HPV. "They found that people who deliberately took a vaccine understood and took actions based on the benefit of having it to live easier lives. If I get more benefit at the same level of risk, what's wrong with that," he said.
Fischhoff does not see adults choosing vaccination to prevent a tick-borne illness as similar to girls receiving a vaccine to prevent a sexually transmitted infection that they could encounter much later in life.
However, he said that at least one study might be more applicable. A 2007 Cochrane Library review found that girls given Plan B emergency birth control to have on hand "just in case" were not any more likely to engage in sex than those who were not given the pills.
The study appears online in the August print issue of the Annals of Behavioural Medicine.