# Stastiticians Help me with this math

203 Views 6 Replies 5 Participants Last post by  boingo82
According to this article 66 of the 200 plus people who have come down with the mumps in the recent outbreak have received the CDC recommended two doses of MMR vaccine. But the article also says that when a person gets the recommended two doses, it is 99% effective in preventing the disease.
http://www.cnn.com/2006/HEALTH/condi....ap/index.html
How does this math work?
It doesn't seem to add up for me.
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but I thought that vaccines were only good for a set period of time? In other words, if you got the MMR when you were 2, about 15 years later it wouldn't be effective.

Are the people who got the mumps kids or older people?
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They had this on NBC last night. They don't know yet what has happened. It could be the vacines have worn off. Eventhough people have received the two.

Oh and this outbreak is much bigger than 200. It is spreading...Its in Iowa, MIssouri and other areas.

http://www.msnbc.msn.com/id/12097831/

http://www.ksdk.com/news/news_articl...?storyid=95233
The math works if we assume that the majority of people have been vaccinated against mumps. Let's pretend that there are 200 cases of mumps in one town where 7270 people were exposed to the illness, and let's assume that 6600 of them were vaccinated and 670 were not. If the vaccine is 99% effective, 1% of the 6600 vaccinated people, or 66 people, get the mumps.

The other 134 people who get the mumps have not recieved the vaccine. Let's say the rate of infection for non-vaccinated people who are exposed is 20% (and I'm just pulling that number out of a hat - it could be far higher or lower). That would mean that in town X, a total of 670 unvaccinated people had been exposed to the mumps.

So, we have 670 unvaccinated people and 6600 vaccinated people being exposed. The vaccinated people are infected at a rate of 1%, and the unvaxed are infected at a rate of 20%. This also means that roughly 90% of the populaton was fully vaxed, and this 90% of the population accounted for about 1/3 of the cases of mumps. The other 10% of the population (unvaxed) accounted for the other 2/3 of the cases.

Clear as mud? It's been a long day... but the difference is because of the different population sizes (vaxed and unvaxed).

dar
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What does 99% effective mean?

It's 99% effective if you never come into contact with a person who has the mumps?

It's 99% effective if you are exposed to someone with the mumps?

It's 99% effective for the first 60 days after the vaccine and then it's not?

I think the whole thing sounds weird. They are claiming that 65% of those who have the mumps did receive the vaccine and that the vaccine is 99% effective. Sounds like a bunch of vaccine propoganda to me.
Dar, you explained it well.
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