Tag Archives: epidemiology

How many cases were there?

My interest is always piqued as soon as a news report refers to rates of some disease being “on the rise”.  I’m usually disappointed as I wait to hear the details since often no actual numbers are provided.

Last week a CKOM news brief declared “Heterosexual sex HIV transmission on rise in Sask“.  Really? No details are provided, just quotes from an AIDS Saskatoon coordinator.

What are the actual numbers?*                    2009        2010        2011        2012        2013

Total HIV cases                                                  94            74             66            55              43

Heterosexual sex transmission                   12            11             12             13               9

Injection drug use transmission**            76            56             49             34             28


The total number of new HIV cases in the health region is falling and this decrease is primarily among injection drug users.  When the percent of cases associated with one cause decreases, the other causes increase as a percent of the total.  In 2009, 81% of HIV transmission was via IDU and 12% from heterosexual sex compared to 65% and 21% respectively in 2013.

Conclusion?  Heterosexual HIV transmission in the Saskatoon Health Region is not increasing.  A larger percentage of cases are the result of heterosexual sexual contact but there are a smaller number of cases.

I also noted that 68% of cases had heterosexual contact with an injection drug user and 30% of cases had sexual contact with a confirmed or suspected HIV+ person.

*Information from the Saskatoon Health Region’s “HIV Strategy Report 2012-2013” and “Better Health for All Series 5: Rates of HIV declining but more needs to be done“.

** Could be +/- 1 since I used a graph to estimate the percentage in each category.




Running for life?

In this week’s health news, running will make you live longer.  Maybe.  Maybe not.

The National Post headline announced that  “Running for five to 10 minutes a day may add three years to lifespans, study suggests“(7/29/2014).  I shouldn’t blame the journalists for exaggerating when the journal article title oversells the study findings as well:  Leisure-Time Running Reduces All-Cause and Cardiovascular Mortality Risk.  (Lee D, Pate RR, Lavie CJ, Sui X, Church TS, Blair SN. J Am Coll Cardiol 2014; 64:472-81.)

The study basics:

  • subjects = 55,137 people who visited a health clinic for a check-up between 1974 and 2002; mostly white, middle to upper class, college educated
  • Data was gathered at the first visit and, where possible, at a second visit whenever that might have occurred.
  • National Death Index used to identify death through 2003.
  • Running was self-reported estimate of activity over previous six months including frequency, duration, distance and speed.

 All of the graphs in the paper illustrate differences in hazard ratio (risk of death in one group compared to another group) rather than differences in actual mortality rates.  The paper provides mortality rates adjusted for baseline age, sex, and examination year so I’ve graphed those rates for comparison.

running and mortality bar chart

We can make predictions based on this information.  Each person-year is observing one person for one year.  So 10,000 person-years could be following 10,000 people for one year, 5,000 people for two years, 1,000 people for ten years or some other combination.  Comparing a group of 10,000 non-runners to a group of 10,000 runners, you would expect to observe 15 more deaths in the non-running group (45 vs. 30).

Another way to describe this is using Absolute Risk Reduction for all-cause mortality.  The ARR = 0.146%.  So, yes, runners did have a 30% lower risk of dying from any cause but the risk of dying was low anyway (0.45% in non-runners) so 30% isn’t really much of a difference.

The big problem with the conclusions stated by the researchers is that association does not equal causation.  They found that runners had lower all-cause mortality.  That does not prove that running reduces mortality.  In fact, in another part of the study, non-runners at the first visit who became runners as of a later visit, did not have lower mortality.  This study observed people who chose to run and chose how much they ran.  Higher duration, frequency, and/or intensity of running did not result in lower mortality.

Which leads to the newspaper headline.  Can running five minutes per day add three years to your life?  Probably not.  The study didn’t even attempt to answer that question.  They estimate that the life expectancy of non-runners  is three years shorter than runners.

This study is not proof that, if you start running, you will live longer.

Unintended consequences.

Sometimes, just when I need it, I get a reminder of something that I’d forgotten.

My last post was in response to movement by some to have age restrictions placed on tanning beds in Saskatchewan.  Click the link to read and you can check out the discussion on Twitter.

So, what did I forget?  When advocating looking at the big picture, I referred to absolute risk of melanoma and the impact of the proposed legislation. I forgot to take another step back and look at the really big picture of all cause mortality.  Often in health research, we discover that an intervention to prevent one outcome actually increases other outcomes.

A tweet from @#JWF# reminded me to do this with respect to this specific topic:

jwf tweetThe study that he refers to was conducted in Sweden and published in the Journal of Internal Medicine.



Tanning beds: Shine a light on some facts.

Doesn’t the Saskatchewan NDP have bigger issues to worry about than banning teens from using tanning beds?

This is one of those issues that where everyone “knows” what they know but few bother to actually look up the facts.

In a well-designed Minnesota study published in the Cancer Epidemiology, Biomarkers, & Prevention, Lazovich et al. reported an increased odds ratio of 1.75 associated with ever use of tanning beds.  Odds ratio is an estimate of risk although it tends to be an overestimate.  This finding is similar to the 2.06 reported in a nested case-control study (an even better design*) from the Nurses’ Health Study.  Based on an odds ratio of 1.75, the risk of developing melanoma is estimated 75% higher for someone who uses a tanning bed compared to someone who doesn’t use a tanning bed.  Therefore, some of the people who use a tanning bed and are later diagnosed with melanoma would have still developed melanoma even if they had not used a tanning bed.

Some numbers:

  • Incidence of melanoma in Saskatchewan in 2003 was approximately 11 cases per 100,000 people (based on 106 cases).  Mortality was 2 deaths from malignant melanoma per 100,000 people.
  • 27% of women ages 16 – 24 in Saskatchewan use tanning beds (from SunSmart Saskatchewan).
  • In the MN study, only 18% of cases used a tanning bed before age 18 and first use before age 18 was not associated with a greater risk of melanoma. (Sorry MLA Chartier.)
  • The incidence of melanoma in the US has been increasing for almost 20 years but the mortality rate has remained relatively constant.

Therefore … banning tanning beds before age 18 would prevent almost precisely zero cases of melanoma.  Doubly true if a teen denied access to a tanning bed just decides to tan outside without sunscreen instead.  Maybe if the person denied access to tanning as a teen never uses a tanning bed after turning 18, a few cases of melanoma might be prevented.

Also, your risk of developing melanoma this year is 0.011%.  If you never use a tanning bed and assuming that the risk estimates in the studies actually reflect the true risk, your risk of developing melanoma this year is 0.008%.  That’s not a huge change in absolute risk.

* A retrospective case-control study compares the exposure rate in people with disease to that in people without disease.  Because people are asking to report exposures from sometime in the past, there is recall bias which affects the estimate of risk.  People with disease are more likely to recall negative exposures.  A nested case-control study uses prospective data on exposure which was collected before disease started which removes the potential for recall bias.