I wanted to do a series of posts on the diseases we commonly vaccinate for, since most people don't actually know that much about them. Mostly this is because we rarely see these diseases, thanks to vaccines! But it's nice to know what you are actually protecting your pet against. To start this discussion I thought it best to go over the basics of vaccine science and immunology. This is unfortunate for me because this was NOT one of my favorite subjects in school, although that was largely due to the poor teaching ability of the immunology professor. Anyway, I'll give it my best shot.
Puppies and kittens obtain their initial immunity from their mother through the colostrum (a special kind of milk produced in the first few days of lactation). This immunity lasts up to around 16 weeks of age, but can start to wane somewhere around 8 weeks. Exactly how long it lasts varies from one individual to another and depends on many factors, some of which we likely don't even understand or know about. Puppies and kittens are unable to respond to any vaccines we give while they are under the influence of maternal immunity. Basically, this means any vaccine given prior to about 8 weeks of age is useless.
Once the puppy or kitten reaches about 8 weeks of age and maternal immunity runs the risk of starting to fail, it's time to start vaccines. Vaccines for common infectious diseases are then given every 2-4 weeks until the puppy or kitten reaches 15-16 weeks of age. Vaccines should be given no closer together than every 14 days as the immune system remains "busy" during this time and will not be able to respond to a subsequent vaccine during that time.
If you have a puppy or kitten that is older than 16 weeks and has never had vaccines (or you are not sure) then an initial vaccine followed by a booster 2-4 weeks later is adequate because at this time there is no maternal antibody to worry about. It is a common misconception that puppies need "X" number of vaccines to be protected. That's not really true, it's all about the age and interval. I frequently see 8 week old puppies from "breeders" who have already had 3 vaccines. Well, that puppy will still need 3 more since the ones the breeder gave were too early. Conversely, the 6 month old stray puppy needs only 2, not 3 or 4 like it would if it came in at 8 weeks old.
This brings up another important point. There are are some animals that don't handle getting multiple vaccines in the same day very well (such as a Rabies and DHPP). Many times we will separate these vaccines (assuming they actually do need both) and do them on different days. It is important to wait at least two weeks between them (not just a week or a few days) because as mentioned before the immune system is busy during that time period and won't respond to the second vaccine if given too soon. Interestingly, you can give multiple vaccines on the same day and get a response to all of them. I can't explain the reason for all this-you just have to take my word for it. People smarter than me have figured all this our for us!
So that's a very basic overview but it will save me from having to go over it all later as we cover diseases. There is some variation in vaccination protocol depending on which disease we are discussing and I'll go over those variations as they come up. Stay tuned!