Effects of climate on Rabies Risk
There is some evidence that hot temperatures can increase the risk of rabies transmission from wildlife to pets and/or humans. As temperatures heat up, wildlife will seek shelter in human structures. A 2006 article in National Geographic notes:
The so-called dog days of summer—a muggy stretch from early July to early September—might also be called the season's bat days in the United States.
This is the period when the flitting critters most frequently turn up
in attics, bedrooms, and camp cabins across the country, sometimes
A recent study from the Centers for Disease Control and Prevention
(CDC) in Atlanta, Georgia, says that 46 percent of U.S. rabies cases
in humans are caused by bites or scratches from infected bats—more
than the 31 percent attributed to dogs.
Five of every six infections from bats occur between July and
September, and the frequency peaks in August.
However, this doesn't mean that the vaccine is less effective in warmer temperatures, just that there's more risk from (unvaccinated) wildlife in warmer climates/seasons. I cannot find any evidence about the effectiveness of the vaccine changing with the climate.
Testing for Effectiveness - serumology
One way that the vaccine's potential effectiveness is tested with with a test called Rapid Fluorescent Focus Inhibition Test (RFFIT). This test measures the body's ability to neutralize the rabies virus. The World Health Organization and the Centers for Disease Control (in the US) have different standards for the minimum protective level measured by this test, and the CDC notes:
There is no "protective" titer against rabies virus. In animal studies, survival against rabies virus infection is often more likely to occur the higher an animal's titer at time of infection, but not a definite indicator of survival. For example in one study of orally vaccinated raccoons 39% of animals with no detectable titer at infection (<0.05 IU/mL) survived, compared to 90% of animals with a titer between 0.05-0.49 and 100% of animals with a titer >0.5 IU/mL.
However, since the WHO's standard for adequate protection in humans is 0.5 IU/mL (source) and the CDC's standard is 0.1 IU/mL (source), that may be one reason that an international vet would error on the side of increased vaccination than risk inadequate protection. It should be noted that these are standards for HUMAN protection and mean
At this level, an immune competent individual would be expected to
mount a rapid response to a booster dose of rabies vaccine in the
event of an exposure, precluding the need of rabies immune globulin
during postexposure prophylaxis.
Recent related research on animals with out-of-date rabies vaccines (the abstract does not state how long) confirms that previously vaccinated animals will mount an adequate immune response to a booster (and thus if exposed should be immediately vaccinated and monitored, not destroyed).
Testing for Effectiveness - Challenge tests
Serum testing does not tell the entire story. An individual may be able to survive rabies even without any detectable rabies antibodies because their bodies remember how to fight the virus. In these cases the only way to determine if the vaccine is still effective is to perform a challenge test.
In a challenge test, an individual is infected with the virus (rabies, in this case) to see how their immune system reacts. Obviously, this is not a test you want to perform on your pet, but it is performed on research subjects.
Challenge testing is how the 3 year rabies vaccine effectiveness was determined (I can't find a source, and the manufacturer's site just says that they have the data on hand). There is currently challenge testing planned for 5 years after the vaccine, and approximately 70% of these dogs no longer have adequate protection as measured by their serum antibodies. Progress can be monitored on the Facebook page for The Rabies Challenge Fund.
In summary, research as to the length of protection given by rabies vaccines is still ongoing and many aspects are not well understood. If you want to extend the interval between your dog's vaccinations over concern of potential side affects (What are the risks/benefits to receiving more frequent vaccinations?) you can request an antibody test, but even if it's negative it doesn't mean that your dog is not protected, just that we don't know.
In addition, insisting on an annual vaccination schedule ensures for the vet that you will bring your dog in for an annual exam (which it does need).
Harder, Ben. "Bat Rabies Threat Rises With Summer Temperatures". National Geographic News
August 15, 2006. link
Moore MC, Davis RD, Kang Q, Vahl CI, Wallace RM, Hanlon CA, Mosier DA. Comparison of anamnestic responses to rabies vaccination in dogs and cats with current and out-of-date vaccination status. J Am Vet Med Assoc. 2015 Jan 15;246(2):205-11. doi: 10.2460/javma.246.2.205. link