The variation in the incidence of whiplash in different parts of the world is considerable. 56 For example, while we found that the population-based incidence of compensated whiplash injury in Quebec was 70 per 100 000 inhabitants in 1987, the incidence rates of whiplash in New Zealand and Australia's State of Victoria were 13 and 106 per 100 000 inhabitants, respectively, during a 12-month period from 1982-83.67 Although the incidence appears very high in Australia, the actual rate of compensated whiplash injury in that region was only 39 per 100 000 inhabitants.67 That is, not all of the whiplash claimants in Australia received compensation for their injuries, whereas in New Zealand and Quebec there were little or no differences between the number of claimants and the number of individuals who received some form of compensation. This means that of the three regions, Quebec had the highest population-based incidence of compensated whiplash injury. Further, since the incidence rates for whiplash injury reported in this study represent the incidence of compensated claims made to the SAAQ by whiplash subjects, the rates are conservative estimates of the actual incidence of whiplash injury in Quebec. Many subjects with injuries of this nature probably do not bother submitting claims for compensation to the SAAQ, leading to an underestimate of the true incidence of whiplash.

Yet, in British Columbia and Saskatchewan, two other Canadian Provinces with single-payer motor-vehicle insurance programs, 68% and 85% of the claims paid out respectively for motor vehicle injuries are for whiplash35,98 compared to 20% for Quebec. For Saskatchewan, with its one million inhabitants and approximately 9,000 claims paid per year, this proportion would translate to a staggering incidence rate of compensated whiplash of over 700 per 100 000 inhabitants, 10 times the rate in Quebec. A possible explanation for this may lie, despite their universality, in the different insurance systems of these two Canadian provinces, namely the no-fault system in Quebec versus the tort system in Saskatchewan.

The incidence of whiplash claims was notably higher among females than among males in most age categories. One hypothesis proposed to explain the consistent observation that the majority of whiplash subjects are female is that given the same head size, men have more neck musculature than women, making them less prone to whiplash injury. 58, 105, 108 Another possibility is that women may be more inclined to file an insurance claim for whiplash than men. The age- and gender-specific incidence of whiplash injury was found not to correspond to the age- and gender-specific incidence of all injuries combined that were reported by police in Quebec in 1987. This distribution shows that males have a higher incidence of all injuries combined than females in each age category and the peak incidence of injuries occurs in 15-24 year-olds. The peak incidence of whiplash injuries, on the other hand, is in the 20-24 year age group.

The regional variation in the incidence of claims for whiplash appears to be dependent on population density and the number of commuters in each region. For example, the regions to the north of Montreal and to the south of Quebec city were found to have a high incidence of whiplash claims. This probably reflects the fact that many people who live in these regions spend a great deal of time commuting to the city for work or recreational reasons. With the increased amount of time and distance traveled, the likelihood of being involved in a collision and sustaining a whiplash injury increases. The northern regions of Quebec had a very low incidence of whiplash claims, probably because these regions are sparsely populated and there is less traffic on the roads, resulting in a lower number of collisions.

The fact that 21.6% of the whiplash subjects did not appear to be injured at the scene of the collision agrees well with the findings reported by Deans et al,18 who found that the onset of neck pain in 22% of collision subjects with neck injuries did not occur until 12 hours or more after the collision. The data showed that longer time to return to activity after whiplash injury was found in subjects with: additional injuries besides whiplash, female gender, older age, greater number of dependents, married/cohabital status, being in a collision involving fatality or severe injury, being in a collision other than rear-end and being in a vehicle other than a car or taxi at the time of the event. Seat belt use was found to result in a shorter absence from activity in contrast with a previous study by Deans et al. 18 The finding that whiplash subjects who were occupants of vehicles other than cars or taxis had a poorer prognosis than those in cars or taxis at the time of the collision may reflect the fact that trucks and other vehicles lack certain useful safety features incorporated into the design and construction of cars and taxis.

The itemized and total costs of whiplash injury paid by the SAAQ calculated in this study have already been reported elsewhere.8,54,58 Relating total costs to duration of absence, the 61.5% of subjects with only a whiplash injury whose absence lasted two months or less accounted for only 15.5% of the total costs, while the 26% whose absence lasted between two and six months accounted for 38.5% of the costs, and the 12.5% of patients still compensated six months after the collision accounted for 46% of the total costs paid out by the SAAQ.