Monday, August 08, 2011

Immunization 411 | VACCINATION Q+A: Medicine looks for herd immunity to protect those most vulnerable to infectious diseases.

By Amanda Mascarelli, Special to the Los Angeles Times |

August 8, 2011 - Healthcare experts warn that vaccination rates have fallen dangerously low in some communities, threatening the "herd immunity" that we depend on to keep us healthy. Concern about this has grown as more parents have been allowed to opt out of requirements that their children get certain immunizations before enrolling in school.

Dr. Michael Brady, a pediatrician at Nationwide Children's Hospital in Columbus, Ohio, chairs the Committee on Infectious Diseases for the American Academy of Pediatrics. He says that when a single child skips a necessary vaccine, there are implications for the entire community. Brady discussed the links between vaccines and infectious diseases in a recent interview.

What is herd immunity?

In safari movies, you see the big herds of wildebeests with the babies on the inside. This is an example of herd immunity — the herd can create a barrier that allows those who are vulnerable to benefit.

How does this apply to people?

If somebody gets infected, that person has the potential to transmit the infection to somebody else. If you could prevent the first infection, then you could theoretically prevent the second. The principle of herd immunity is that if you have a large enough group of people protected — either because they were vaccinated or previously had the disease and became immune — when an infection gets introduced into a community, you would reduce the likelihood of having further transmission.

We have some people who unfortunately cannot be immunized. This includes those whose immune systems are not strong and those who do not respond very well to the vaccine. If we can get very high levels of vaccination in all of those who can receive a vaccine, the people who may remain vulnerable can be protected.

Are there signs that herd immunity is faltering?

There are infections like measles, chicken pox and rubella that we don't worry about too much anymore in the U.S. But because of globalization, they are actually coming across our borders on a regular basis.

We've recently started having outbreaks of measles. If you have a community where the measles immunization rate is 95% or higher, it is very unusual for us to see outbreaks. In states where they have very liberal personal exemptions from the school vaccine requirements, we have a lot more outbreaks.

Is measles the only example?

We also saw this in California last year with pertussis. They had many under-immunized children. The zero-to-6-month-olds who have not yet developed full immunity from their pertussis vaccines are typically protected by herd immunity. These infants could not take advantage of that because there was so much transmission among other age groups who were under-immunized.

Is there concern that herd immunity could break down on a much larger scale?

One thing that has been very helpful in the past has been the school vaccine entry requirements. But in response to anti-vaccine groups, more and more states are getting pushed to make the recommendations more lax. Legislators are saying, "Why don't we just let them say no?" That not only results in a lack of protection for the unimmunized child, it also creates a larger hole in our herd immunity.

Most people are lax about getting their annual flu vaccine. Who needs it the most?

We have certain people who get sick and certain people who are responsible for a lot of the transmission. People who get sick are the young children, people with chronic diseases and the elderly. The people who spread it are the school-age children and young adults. So the people who need it for their health and the people who spread it are different groups, but when you put them all together, it is everybody.

Why do we need to get immunized every year?

There are two reasons. One, there is typically enough change in the virus strains that immunity from prior years would not be effective. In addition, even if they are the same, the immunity from the vaccine seems to wane within about a year or so.

When is the best time to get a flu shot?

In the U.S., the flu season typically starts between November and February and ends sometime between March and May. You'd like to make sure everyone gets immunized before any potential start, which means that the optimal time would be sometime in late August through the beginning of November.

A lot of people don't take the flu very seriously. Why is it such a serious health concern?

It is not a fun illness to have. If you have something that lasts just 24 hours, that is not the flu.

When you get influenza, you are really very sick. Each year thousands of people die from influenza and its complications. Most of those who die are the most vulnerable, such as young infants, the elderly and those with chronic diseases, particularly chronic heart and lung disease. However, recent influenza seasons have made it clear that even those who were previously healthy can become very sick and even die. Influenza is especially severe in women who are pregnant.


This interview was edited for space and clarity from a longer discussion.

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