Picture this: An owner takes their dog to a veterinarian for routine vaccines. The veterinarian recommends a DAPP vaccine (distemper, adenovirus, parvovirus, parainfluenza), as well as vaccination for rabies and leptospirosis. After some discussion, the owner declines the lepto vaccine, but proceeds with the others. The next day, they call back after doing some reading, having decided they’d like to have the dog vaccinated for lepto after all. When can the dog receive the additional vaccine?
A lot of veterinarians would likely recommend waiting at least two weeks, because that’s the minimum interval for boosters for many of our canine vaccines. It’s a pretty well entrenched dogma that’s been propagated through teaching, continuing education talks and reviews for years, but if you try to trace back that recommendation, you eventually hit a dead end.
- It’s sometimes stated that the immune system can get overwhelmed or overloaded if too many vaccines are given close together, but there’s no hard evidence that happens. People and animals are bombarded by antigenic stimuli every day, so out immune systems are designed to be ready to go 24/7.
There’s actually no clear immunological basis for a universal 14-day minimum interval between administration of different vaccines (vs boosters of the same vaccine) in dogs and cats, or that giving different vaccines closer together than this results in clinically relevant vaccine failure. In human medicine (where they have a lot more data), there’s no recommendation for a minimum interval between most vaccines, with a few exceptions that don’t apply to pet vaccines.
- According to the US CDC guidance on Timing and Spacing of Immunobiologics: “There is no evidence that non-live vaccines interfere with the immune response to other non-live vaccines or to live vaccines. Any non-live vaccine can be administered either simultaneously or at any time before or after a different non-live vaccine or live vaccine. The 2 exceptions are a 4-week interval between PCV13 and MenACWY-D in a person with anatomic asplenia and the separation of doses between PCV13 and PPSV23. (These exceptions are not applicable to dogs and cats of course).
- Regarding live vaccines, the CDC guidance goes on to say: Limited data are available regarding interference between live vaccines used in the United States. The immune response to one live-virus vaccine might be impaired if administered within 28 days (i.e., 4 weeks) of another live-virus vaccine. In a study conducted in 2 U.S. health maintenance organizations, the risk for varicella vaccine failure (i.e., varicella disease in a vaccinated person) among persons who received varicella vaccine within 28 days of MMR vaccination was threefold higher than among persons who received varicella vaccine >28 days after MMR vaccination. Another study determined that the response to yellow fever vaccine is not affected by monovalent measles vaccine administered 1–27 days earlier. The effect of nonsimultaneous administration of rubella, mumps, varicella, and yellow fever vaccines is unknown.”
Based on this, the concern is only with serial administration of multiple injectable modified live vaccines, and it likely only affects some of these vaccines, not all of them. Combination core vaccines for pets (DAPP for dogs, FVRCP for cats) include modified live components, but the other vaccines we use (for lepto, Lyme disease, feline leukemia, and some rabies vaccines) are non-live vaccines. There are some modified live feline chlamydia vaccines, but those are typically part of FVRCP vaccines. (If there was a chlamydia-only live vaccine, I’d consider spacing it by at least 4 weeks from an FVRCP vaccine, but I don’t know if that would actually be necessary). We don’t otherwise need to worry about giving different modified live vaccines close together because the products don’t exist.
So in my opinion the dog in our initial scenario can be vaccinated for lepto at any time with no additional concerns about “interference” or poor response because of the vaccines the dog received at the initial appointment. If the owner is told to wait, there’s a good chance life will get in the way and a later appointment won’t get booked. I wouldn’t want to miss an opportunity to vaccinate the dog, so I’d have them come back as soon as they are able. That also helps the dog get protection as soon as possible.
Now let’s think about non-injectable vaccines. Picture this instead: Same dog, same owner, same recommendations. This time the owner calls back the next day and says they will be boarding the dog later this year, so they want to get it a “kennel cough” vaccine. You recommend a mucosal (oral or intranasal) vaccine, since they are more effective than injectable kennel cough vaccines. When can the dog receive an oral or intranasal vaccine?
Mucosal (oral, nasal) vaccines that we use for “kennel cough” (typically Bordetella bronchiseptica, canine parainfluenza virus, canine adenovirus in various combinations) in dogs are modified live vaccines, so there might be some issues to consider with regard to minimum vaccination intervals, but we have a complete lack of data on the impact in pets.
- In humans, the CDC guidance says “Two or more injectable or nasally administered live vaccines not administered on the same day should be separated by at least 4 weeks, to minimize the potential risk for interference. If 2 such vaccines are separated by < 4 weeks, the second vaccine administered should not be counted and the dose should be repeated at least 4 weeks later. On the day a live injectable or intranasal vaccine will be administered, providers should ensure that no live injectable or intranasal vaccine was given in the previous 28 days.”
This is the one situation where there might be a biologically plausible argument for a minimum interval between different vaccines, but whether it actually has a clinically significant impact is another question. We have no idea if it’s an issue for pets, but this scenario could occur with dogs given a modified live core vaccine and then a modified live intranasal or oral vaccine less than 4 weeks apart. I’ve never really considered this to be a big concern, as I’d typically be more concerned about missing the opportunity to protect the dog against a vaccine-preventable disease, but I don’t think we can ignore it completely.
So in the scenario with the newly vaccinated dog and the owner now requesting a kennel cough vaccine, I’d consider how important the vaccine is in the very short term and over the next several months. If the dog won’t be boarded for a few months, I’d recommend waiting 4 weeks for the kennel cough vaccine out of abundance of caution, to try to get the best response possible on the first dose. If the dog is going into a high risk environment (e.g. boarding) soon, then I’d give the vaccine right away, but also recommend another dose 4 (or more) weeks later based on the theoretical potential for a poorer response to the first dose. That may be overkill, but it’s the one scenario with a plausible basis for setting a minimum dosing interval between different vaccines.
To sum up another very long blog post:
- There is no scientific basis for a minimum interval between giving different non-live vaccines to dogs and cats.
- The only scenario in which a minimum interval between different vaccines could plausibly matter is sequential administration of multiple modified live vaccines, which doesn’t come up very often in pets anyway, and the potential for interference it based only on extrapolation from human medicine, with no evidence that it is clinically relevant in dogs or cats.
This also raises questions about vaccination of pets in shelters. Shelters vaccinate frequently using modified live vaccines, including kennel cough vaccines for dogs. Does that mean those dogs may be suboptimally vaccinated in some situations, OR can we consider shelters a natural model that shows this kind of interference actually isn’t a concern? An interesting point for discussion, but shelter vaccination strategies are fodder for a separate post.
Don’t forget to check out the new paired resource from the Ontario Animal Health Network (OAHN): Vaccination Timing and Intervals in Dogs and Cats (infosheet and infotable). If you don’t have an OAHN login, veterinarians and RVTs can sign up for free.