06/28/2013 12:00 pm ET Updated Dec 06, 2017

Does the 2013 US Childhood Vaccine Schedule Really Recommend 27 Shots Before 2 Years of Age?

This question originally appeared on Quora.
Answer by Robyn Correll Carlyle, MPH

The schedule recommends 27 doses of vaccines, but that doesn't necessarily mean a child will receive 27 shots.

Combination vaccines are available that contain several recommended vaccines in a single shot to reduce the number of injections a baby will need. Pediarix, for example, combines three of the recommended vaccines typically given together: DTaP (Diphtheria, Tetanus, and Pertussis "whooping cough"), Inactivated Polio Vaccine (IPV) and Hepatitis B vaccines. Combination Vaccines

Not all vaccines are injected. The vaccine against rotavirus, for example, is an oral vaccine. Page on Cdc Before the U.S. switched to the inactivated polio vaccine, the live-attenuated polio vaccine (OPV) was administered by mouth as well. That oral polio vaccine is still used in many other countries, but it is no longer used in the U.S. Oral polio vaccine (OPV) 

Image from: Why Flu Vaccines Are Important For Children And Parents

So ... you might be wondering "Why so many doses?"

Not all vaccines are made the same way.

Some vaccines are made using the whole pathogen, or the thing that causes disease like a virus or bacteria. The pathogen is weakened in the laboratory until it's a pathetic, withering (but just barely recognizable) version of itself.

These are known as live-attenuated vaccines. Because these vaccines are still alive, they can replicate when they enter the body, and they provide the closest we can get to the "natural acquired immunity" -- without the getting really sick part. With live-attenuated vaccines, you really only need 1-2 doses to get immunity for the rest of your life.

But because these vaccines use a live pathogen, there are certain risks. They have more potential side effects, like slight fever or malaise, and there is the super, extra-rare chance in a blue moon that they will cause the disease. When polio was common in the U.S., for example, a live-attenuated vaccine made sense. The benefit of stopping polio far outweighed the rare risk of the vaccines. But when we successfully eliminated polio in the U.S., we noticed that nearly all of the cases of polio were because of the oral vaccine. So we switched to the inactivated vaccine.

Inactivated vaccines use the whole pathogen, but they are already dead. Killed in the laboratory. They can't replicate and they aren't alive, so they don't have the same risks as the live-attenuated vaccines. But, as a consequence you often need more doses and booster shots to develop and maintain immunity.

Sub-unit vaccines are made using only the part of the pathogen that will jumpstart the body's immune system -- not the whole thing. The HPV vaccine for example uses just the outer protein coating of the human papillomavirus. Risks and side effects of this type of vaccine are really rare. But because it can't replicate, more doses are required to get an immune response.

Sometimes, it's not the pathogen itself that hurts the body, but the toxin that the pathogen produces. Toxoid vaccines protect against the damaging effects of toxins made by diseases like tetanus and diphtheria. But much like inactivated and sub-unit vaccines, toxoid vaccines require multiple doses and boosters for sustained protection.

So why so many doses?

Because we're now able to protect against 10 diseases that used to cause massive amounts of disability, brain damage, and death in children.

Because we're using methods of producing vaccines that carry fewer risks but require more doses.

And because that is what research has shown to be necessary to develop a strong enough immune response to protect the child through those critical first two years of life.

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