Department of Companion Animal Clinical Studies, Faculty of Veterinary Science, University of Pretoria, South Africa
Vaccines are the most effective public health tool ever created. Human life expectancy has been extended by 30 years since 1900 and it is estimated that 25 of these 30 years are attributable to public health advances, most notably vaccination. There is a threatening minority that for various reasons have become vocal against vaccination and these voices have raised concerns, cast doubt and spread misinformation and misconception regarding vaccination. Left unaddressed, this could lead to a public decline in the confidence in vaccination. This decline could lead to reduced public uptake in vaccination and a number of diseases, now well under control, may re-emerge. Vaccine coverage rates have achieved record highs and with this, infectious diseases that just a generation ago lead to high morbidity and mortality, are no longer a threat to the human population. The classic example of a study that was seriously flawed was published in The Lancet by Wakefield et al. in 1998 and subsequently completely withdrawn, is a paper which purported an association between the measles, mumps and rubella vaccination in children and autism.1 The author of this paper was found guilty of serious misconduct and The Lancet retracted the publication in full and found that portions of the data reported had been falsified. This single report has resulted in widespread distrust and scientism in vaccination as a whole and resulted in reduced vaccine uptake and subsequent disease outbreaks in the West of measles in particular where these diseases were up until recent times almost unheard of. Easy public access to the internet and the many and varied opinions expressed there have driven a form of hysterical hype in some quarters around the safety of vaccines.
Human scepticism regarding the safety of vaccines is not restricted to the human population but has resulted in scepticism in general and the pet population may fall foul of these misconceptions. Particularly for disease like distemper and rabies, the repercussions go far beyond just the pet population. In the case of distemper, invaluable wildlife would be subjected to ever-increasing risks and in the case of rabies, the human population becomes subjected to increased risks of infection from dogs (the single most important vector of the devastating disease).
The efficacy of a vaccine in preventing disease in a population depends on what proportion of the population is vaccinated. This effect is described as herd immunity. When the majority of the population has been vaccinated against a contagious disease, this provides a measure of protection for those individuals who have not been vaccinated. This means that the chain of infection is disrupted and the greater the proportion of vaccinated individuals there are in a population, the greater the disruption to the chain of infection and hence the more efficient the intervention in controlling disease or the smaller the probability that an unvaccinated (susceptible) individual will come into contact with an infectious individual. It quickly becomes clear that the greater the uptake of a particular vaccine within a population, the more effective the vaccine is at preventing the spread of the infectious disease.
A helpful epidemiological principle to understand in this regard is known as the basic reproductive number (denoted R0). This figure defines the number of cases one case of particular infectious disease can generate over the time course of its infectious period. A low R0 (R0 < 1) means that a single case of an infectious disease placed within a population will infect less than one other individual and hence the infectious disease quickly dies out as it fails to propagate within the population. Diseases with high R0 values are highly infectious and propagate quickly within susceptible populations. If a disease should have an R0 value of 10, a single case of this disease will infect 10 other individuals within that population and the disease will spread quickly within a population. The benefit of knowing what a particular diseases R0 value is, is that knowing this statistic allows one to determine what percentage of a population needs to be vaccinated to prevent disease outbreaks and to eventually eradicate a contagious disease. Human measles has an R0 of 12–18. This is exceptionally high and preventing disease outbreaks is only possible if close to 100% of the human population is vaccinated. Canine distemper virus probably has an R0 in the same ball park. Achieving such high coverage obviously requires much energy and a cooperative informed population. Rabies, on the other hand, has an R0 of only just above 1. This means that to achieve good rabies control in a dog population requires around 60% coverage with a vaccine which should be reasonably easily achievable. Despite this, rabies control has remained elusive. The main reason for this is the rapid turnover of the dog population in the rural feral population where rabies is the biggest problem. The dog vaccinated today has a short life and in that short life produces several litters of puppies. This means that vaccine drives in these circumstances have to be very frequent to even hope to maintain around 60% of the population vaccinated at any one time.
Despite their tremendous benefit, vaccines may have unwanted side effects. There is tension between the risk of infection, the owner's financial means, the protection provided by the product and the chance of an adverse reaction to a vaccination. Vaccine-induced illness has become an issue despite its prevalence being very low. The reason for this is that in some cases the occurrence of adverse reactions supersedes the occurrence of the disease being vaccinated against. If routine standard vaccination protocols are blindly applied, some dogs will be vaccinated too frequently or for diseases they are not at risk for. In some instances the risk of disease is so high that more frequent vaccination may be prudent.
That puppies and kittens need 3 initial vaccinations ending at 16 and 12 weeks respectively, is not generally in question. Neither is the use of a booster vaccination a year later. It does however appear that subsequent to this, vaccinations can safely be repeated every 3 years. The recommendation for annual vaccination was decided arbitrarily in the late 1960s as an educated guess by experts in the field. The assumption was that annual vaccination would not hurt and would provide an opportunity for annual health checks on pets. And thus, without any sound scientific evidence in its support, the practice of annual vaccination has continued. This practice is now under question. It seems clear from the evidence that immunologically there is no need for annual vaccination for the core canine and feline vaccines after the first few years of use. It is crucial, however, to remember that there is more than immunology at play here. In the South African (and indeed any developing world region) where the intergeneration gap is short, the majority of the dog and cat population may well be feral, and the animal-owning population do have a culture of vaccination, it may well be irresponsible to reduce the pressure to vaccinate annually.
The benefits of vaccination for serious diseases far outweigh the danger of vaccine-associated complications. In some senses we have become victims of the success of vaccination. Vaccination has however reduced many serious diseases to negligible levels in some cases, because of this, vaccine-associated side effects have had a growing impact on the perceptions of vaccine safety when risk benefit ratios are considered. What many fail to factor into their assessments of vaccine safety is the effect of reduced herd immunity in a population to levels that leave significant proportions of a population fully susceptible to disease and the possibility of devastating disease outbreaks in these populations.
A study in the UK evaluated adverse reactions to vaccines between 1995 and 1998.2 A total of 3188 reactions were reported. Of these only 7% could be deemed to be vaccine associated, 18% possibly related, and 73% probably not related to vaccine use. Even if all adverse events reported were regarded as vaccine associated, this would still only amount to 0.004% of the 6–8 million doses of vaccines administered to dogs and cats in the UK annually. A similar study was performed in the USA.3 Just over 1.2 million dogs vaccinated at 360 veterinary facilities over a year were evaluated for allergic reactions, urticaria, or anaphylactic reactions within 3 days of injection. Nearly 4700 adverse events were associated with the administration of almost 3.5 million vaccine doses to just over 1.2 million dogs (38.2 reactions per 10 000 dogs vaccinated). The risk of an adverse event increased significantly with neutering, young age (less than 3 years), and the number of vaccines given per visit.
1. Wakefield AJ, Murch SH, Anthony A, Linnell J, Casson DM, Malik M, et al. Ileal-lymphoid-nodular hyperplasia, non-specific colitis, and pervasive developmental disorder in children. The Lancet. 1998;351(9103):637–641. (Retracted).
2. Gray AK. Cat and dog vaccination: results from the Suspected Adverse Reaction Surveillance Scheme. Vet Rec. 1998;143(16):455.
3. Moore GE, Guptill LF, Ward MP, Glickman NW, Faunt KK, Lewis HB, et al. Adverse events diagnosed within three days of vaccine administration in dogs. J Am Vet Med Assoc. 2005;227(7):1102–1108.