The Pertussis Outbreak in California (Updated)

As I talked about a few weeks back, California has been experiencing an outbreak of whooping cough (pertussis) over the past two or three months.  For a while there were a lot of articles and blog posts about how this outbreak was the fault of the "anti-vaxers" but fortunately these articles have mostly died down.

However, these articles and posts were replaced by ones calling this the "worst outbreak in 50 years".

As I have pointed out a few times, there are likely many reasons why we are seeing increases in the number of cases. The reasons range from  problems with the vaccine not working as well as it could to the fact that the vaccine only lasts for about a decade before it no longer provides protection to the fact that the outbreak in California might be occurring in groups that don't have good access to health care.

Regardless of the reasons, an epidemic of whooping cough has been declared in California. But we are really in the midst of the "worst outbreak in 50 years" or is something else going on?

To answer that question, I looked at the numbers published by the CDC in the past several years to see how the current year compares to previous years. I took the total number of provisional pertussis cases reported week by week for the first 7 months of the year for 2004, 2005, 2009, and 2010. I selected 2005 because that was the year of the last whooping cough outbreak and 2004 and 2009 are included as comparison points.

I think it is worth noting that the numbers that the CDC publishes can be different that the numbers that you here from other sources. For example, one source from several weeks back says that as of June 30th there were 1,337 cases reported in California yet the CDC's most recent published data only shows 809. The differences between the numbers could be caused by a number of factors such as how a case is confirmed (some tests are more accurate than others) or any delay in reporting cases to the CDC.  But I don't think the differences are significant here because it is the trends that I am looking at, not the absolute numbers.

So, without further ado, first up we have California.

As you can see on the chart above, there are clearly more cases at this point in the year than in any of the prior years included. There is no doubt that California is having a epidemic of whooping cough. However, the total number of cases have basically been flat for the past 3 weeks, so perhaps the outbreak is already winding down. Only time will tell for certain.

But that is California, what about the country as a whole?

This chart tell a different story. For the country as a whole, we are experiencing a milder year this year than the other years included. The year to date numbers are slightly below the numbers from 2004 and well below the numbers for 2005 and 2009. So when you look at the entire country, we are clearly not having the worst year in 50 years.

So what does the state by state picture look like? The following chart is showing the total number of cases for 2009 and 2010 on a state level as of July 31st.

It may be a little hard to see on the chart (click on it to see a larger version) but more than half of the states are showing less cases than last year while only a few are showing more. This tells me that while there are areas of the country that are having outbreaks the rest of the country is doing better than it did last year.

Overall, the picture is mixed.  There are a few states that are having outbreaks but the rest of the country is doing pretty good.  Only time will tell what the final numbers of this year are but it isn't shaping up to be the worst in 50 years.

P.S. If you live in an area that is having an outbreak, come into close contact with young children, and it has been over ten years since your last shot, you may want to talk to your doctor about getting one.

UPDATE
If figures.  The data stays basically flat for three weeks so I think it is safe to write about this topic and then the CDC goes and publishes a new data point that changes what I just got done saying.

The new figures for the week ending Aug 7th show a massive increase in the total number of cases in California.  The prior weeks total for California was 809 but the new total number is 1,964, a staggering increase.  However, the number of new cases for the current week was only 17.

I guess that most of these cases were for past weeks and just now percolated into the CDC's numbers but I have to admit I am a little confused.  The definitive test for whooping cough takes about a week to get results back and the data for California has been flat for the past three weeks.  So either there is a backlog in testing, a significant delay in reporting to the CDC, or something else going on with the numbers.

The new total number for the US also jumped from 7,781 to 9,412, - a change of 1,631 - with a weekly change of 210.  Most of this total change was driven by the California change (1,155) and by a jump in North Caroline from 0 to 123 cases.

The new number means that the total number of cases in the US are now above 2004 and 2009 but still below 2005.

On the state by state level, the picture does not change too much.  Well over half of states are still showing a decrease from last year and only two more states switched from basically unchanged to showing a noticeable increase.