FAQ: Why Does Disease Still Happen Even When We Vaccinate?

Here are two common questions about vaccinations and the spread of disease answered by ASPCA Shelter Medicine Services veterinarians.

Q: We know that timely vaccination is one of the most effective tools we have to protect the health of shelter animals. But why do we still have disease if we are vaccinating the animals?

A: Here’s a basic fact about how diseases spread: When a certain percentage of the population is immune—they have been vaccinated against a contagious disease—that disease can no longer spread through the population and cause an outbreak. Individual cases may still develop, but they will be isolated and small in number.

How much of the population needs to be vaccinated depends on the specific infectious organism, but it is usually fairly high. This is one reason why it is a standard recommendation for all dogs and cats, regardless of lifestyle, to be vaccinated against canine distemper virus, canine parvovirus and feline panleukopenia virus.

We must choose and use the vaccines available to us strategically in order to best protect the most animals. The recommendation to vaccinate all (or nearly all) animals immediately as they enter a shelter is completely based on the concept of herd immunity. In a big, high-density situation, the risk of exposure to life-threatening disease is high, no matter how expertly a shelter medicine program is run.

At the same time, we know there will be a population of individuals that are unprotected and thus susceptible to disease. This is simply the effect of a big population of animals comingling.

Q: In the last three weeks we have had six dogs diagnosed with parvo. There was a sick puppy who came to the shelter about a month ago who may have brought the virus in. We are still vaccinating everyone when they arrive at the shelter and giving boosters if they stay with us.

We are cleaning the heck out of things and doing our best to keep the animals separated, but we are really worried—a couple of these pups already had three shots before they got sick, including one who just came back from foster for an adoption event. Shouldn’t they have been protected?

A: You may have heard that two, three or even four vaccines is the magic number that will keep puppies or kittens safe. Unfortunately, this isn’t the case. Protection comes not as a result of the number of vaccinations that are given, but the timing. We consider youngsters to be reliably protected only when they receive their core vaccines after a certain age, regardless of the number of times they have been given booster shots.

When young animals nurse from mom in the first days immediately following birth, they will receive mom’s antibodies for whatever diseases she was previously vaccinated against or actually infected with. These maternal antibodies are tremendously helpful and provide critical protection for puppies and kittens for the first several weeks of life before they eliminated from the body. However, maternal antibodies also block vaccines from working.

Maternal antibodies wane over time and the levels will be different in individual animals, even within a litter. We know that levels will be low enough to allow a response to vaccination by about four months of age in nearly all animals. But before that age, it’s a guessing game. Levels may be high enough to prevent the puppy’s or kitten’s own immune system from responding to the vaccine, but not high enough to protect against disease. If exposure to disease occurs during this critical time, this can lead to the scenario described in this email.

These puppies were all between 10-14 weeks of age—right in that window of susceptibility when maternal antibody levels are too high to allow effective vaccination but too low to protect from disease, so when they were exposed to that one sick pup they unfortunately fell ill as well.

Remember, we can’t rely on a certain number of vaccines as an indicator of protection because there isn’t a magic number. We just need one vaccine given at the right time, and the key to that right time is individual to the animal. Maternal antibody interference is a significant concern, and it’s considered to be the most common reason for apparent vaccine failure in both dogs and cats—far and away the most likely explanation for a scenario like this, rather than a new or resistant strain of disease.

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