Saturday, December 29, 2007

Feline Vaccines and relative risk

Towards separating the chaff from the wheat

Blogging on Peer-Reviewed ResearchThis 2007 retrospective cohort study “Adverse events after vaccine administration in cats: 2,560 cases (2002-2005)”comes from the University of Purdue School of Veterinary Medicine in association with Banfield veterinary hospitals. It evaluated the incidence of vaccine associated adverse events (VAAEs) diagnosed within 30 days of vaccination in cats and attempted to determine risk factors for their occurrence. This is one of the largest studies of its kind to date reviewing 496,189 cats vaccinated at 329 hospitals. The study followed these cases for up to two years post incident and revealed pertinent information for practitioners.

The general conclusion of this study reinforces the increasing data base that confirms VAAE rates are actually very low. Additionally, it found no association between vaccinations and an increased incidence of tumor associated vaccine reactions- a relation that has been long overblown by anti-vaccination advocates.

To be clear, vaccinations work and thus have the potential for adverse reactions including the possiblity in cats of being associated with vaccine associated tumors (although even if you assume a link, the national incidence appears low at approximately 1 to 2 sarcomas/ 10,000 cats).

What is not made clear or somehow seems to be buried away in the publics statistical ignorance is that a growing number of studies are revealing that vaccine related adverse reactions are indeed rare and when they do occur, they are overwhelmingly mild reflecting observations seen across many animals including humans. Yes there are rare potentially devastating effects that need to be carefully considered when developing what should not be forgotten is the ultimate strategy- that of of protecting innumerable animals from very common and far more devastating diseases.

The Purdue/Banfield study came up with some interesting data that may influence general practitioners towards modifying vaccination protocols slightly in light of these findings. They conclude that “Although overall VAAE rates were low, young (below one year) adult neutered cats that received multiple vaccines per office visit were at the greatest risk of a VAAE within 30 days after vaccination. Veterinarians should incorporate these findings into risk communications and limit the number of vaccinations administered concurrently to cats.”

Basically, it seems that when problems are noted, they tended to occur more frequently in younger neutered cats given multiple vaccines. The authors also note “…the factor associated with the greatest increase in VAEE risk was the number of concurrently administered vaccines or the total vaccine volume administered during the office visit.” This is consistent with another more limited study done in dogs where a vaccine/volume association was found in animals less than 10kg. The canine study by the way also indicated a very low adverse incident rate.

This information can be used to fine tune vaccination protocols and further reduce already low risks. For example, a simple strategy for the veterinarian here would be to split vaccine doses in young cats and assess each individual animal based on the risks/benefits. Vaccine producers can take this information into account when designing vaccinations, especially in light of the probable development of future vaccines against serious feline disease.

Incidentally, the reactions noted consisted of two types. One was characterized by a nonspecific systemic reaction with clinical signs of anorexia, lethargy, fever, or general soreness. The other consisted of a local (injection-site) reaction. As mentioned, this particular study found no association with vaccine related tumors through the study range of one to two years post vaccination. Other longer term studies will help continue to reduce the gaps in our knowledge regarding potential vaccine related effects. Anti vaccine advocates will surely continue to take a “God in the gaps” approach and insist vaccinations cause any number of maladies while ignoring the obvious huge benefits they bring.

The authors state that “Vaccinations are the most cost- effective method of preventing infectious diseases, but VAAEs can sometimes raise more concern among veterinarians and cat owners than do the diseases the vaccines are intended to prevent.” This study adds to the increasing understanding of where our priorities should be and helps quiet the loud fear mongering anti-vaccine advocates by countering their claims with facts.

The goal here, as in all medicine, is to understand all that we can about a particular intervention and balance the risk versus benefits to provide the most reasonable and effective protection. This animal study serves to illustrate how practitioners evaluate real evidence to critically examine and re-examine their ability to improve, modify, or fine tune whatever intervention is being examined- feline vaccinations this case.

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