1. Replacement of Td with Tdap in Michigan in 2006
Kyle S. Enger, MPH
Vaccine-Preventable Disease Epidemiologist
Division of Immunization
Michigan Dept. of Community Health (MDCH)

2. Td and Tdap
Td boosts tetanus and diphtheria immunity (given to y. olds; then every 10y)
Td is required for Michigan students
if > 10 y since last tetanus iz.
Tdap is Td with pertussis added
FDA-approved: mid-2005
VFC in MI: Sept. 2005
ACIP-recommended (MMWR): March 2006

3. Data source: MCIR (Michigan Care Improvement Registry)
Operational since 1998
Birth-to-death immunization registry
< 20 y age restriction removed June 2006
~95% of providers participate
Part of CDC’s IIS sentinel site project
Michigan’s population is large
10 million people
130,000 births/year
We’ve been a lifespan registry for less than 1 year.

4. Td & Tdap by month,
Td reports to MCIR were largely replaced by Tdap in 2006.
This chart shows the number of Td-containing immunizations administered each month since 1997.
There was a Td shortage in 2001; after that, Td trends stabilized, peaking every August at close to 25,000 immunizations per month.
We start to see Tdap in the MCIR in mid-2005, but it really takes off in summer About 30% more Td-containing immunizations were recorded in 2006 compared to 2005.
However, some of this increase is due to the removal of the <20y age restriction on MCIR.

5. Td & Tdap by year,
You may wonder whether the increase in immunization that we saw in 2006 is due to Td-immunized people returning to get Tdap. This chart indicates that this effect is minor.
The tall bars show the number of Td-containing immunizations that are the 1st recorded for a given person. The smaller bars are additional immunizations.
This chart shows that relatively few people in MCIR have >1 Td-containing immunization recorded. However, older people are less likely to have data in MCIR, so we may lack information on people’s earlier Td-containing immunizations.

6. Td & Tdap, age dist.
Now we are examining the age at which Td-containing immunizations were given.
The age distribution is quite stable from 1997 to However, we can see that the share of reported immunizations given to persons over 20 years is increasing.
It is a bit difficult to see here, but if persons aged 20+ years are removed, the share of immunizations given to year olds increased every year from 2003 to 2006.

7. Td & Tdap coverage by age group
Despite impressive numbers of Td and Tdap vaccinations being recorded every year, the MCIR still reports low coverage levels.
Td coverage peaks at about 50% in year olds.
Tdap coverage peaks at 16% in 12 year olds. It’s easy to see here that Tdap has been spread widely across the adolescent age groups.
Although the MCIR is now a lifespan registry, it will take time to accumulate data about adult immunizations.

8. A tale of two counties Kalamazoo County Birth cohort of 3200
City of Kalamazoo
62,000 population
Lower cost of living
Washtenaw County
Birth cohort of 4200
City of Ann Arbor
100,000 population
Higher income
Although no two counties are alike, Kalamazoo and Washtenaw counties are similar in many ways.
Both in southern Michigan
Both have strong local health departments
Both have large state universities

9. Kzoo vs. Washtenaw
This chart shows the number of Td-containing immunizations given in each county from 2002 through 2006.
Kalamazoo is in warm colors, Washtenaw is in cool colors.
Despite Washtenaw’s greater population, Kalamazoo reports much more Tdap in 2006.
Kalamazoo also peaks 2 months later in 2006.

10. Pt. cases by county, 2005 & 2006
As you probably suspected, Kalamazoo suffered a large pertussis outbreak in 2006.
This chart shows the number of pertussis cases in Michigan counties in 2005 and Both Washtenaw and Kalamazoo are on the far right side of the chart, but Kalamazoo had more than half of Michigan’s pertussis cases in 2006.
Some of Kalamazoo’s surge is real, but some is attributable to intensified surveillance.

11. Td & Tdap coverage by age group
Now we’ll look at immunization coverage in Kalamazoo and Washtenaw counties. For comparison, here’s the statewide coverage chart again.
Pay particular attention to the blue bars, which are Tdap.
Note that it tops out at about 17% for Tdap in 12 year olds.
Gov’t-purchased Tdap was not available for 18+ year olds until Jan in MI.

12. Washtenaw Td/Tdap covg.
This shows Td and Tdap coverage for Washtenaw County. The format is exactly the same as the statewide coverage slide.
Tdap coverage in Washtenaw is higher in young adolescents, but remains low in older adolescents.
The steep drop in Td coverage at age 18 is because Washtenaw has a large university population, so coverage is driven downward because the denominator is inflated ( y vs y), and because students are entering Washtenaw from out of state.

13. Kzoo Td/Tdap covg.
Kalamazoo’s Tdap coverage is higher still. Even so, it tops out at 30% for 12 year olds.
It’s roughly double Washtenaw’s coverage for teens aged 15+, though.
Kalamazoo commented that ‘the disease basically marketed the new vaccine’.

14. Pt. epi curve, Kzoo
This is the epi curve for the Kalamazoo County pertussis outbreak.
Note the large increase in week 35, corresponding to the beginning of September.
It peaked in late September, which probably explains Kalamazoo’s late peak for Tdap administration.
Washtenaw County’s epi curve indicates only sporadic cases, similar to Kalamazoo County in 2005.

15. Pt. age dist. pie charts
Compared to pertussis cases throughout the rest of the state in 2006, Kalamazoo had more school-age cases; 50% aged 5 to 17.
Relatively little disease was seen in persons aged 18 and up.
It’s unclear how large an impact Tdap might have had on this outbreak. Many of the children affected were too young to have received Tdap, and immunization didn’t spike until late in the outbreak.
However, one school district required Tdap immunization of all teachers & students.

16. Kyle Enger engerk@michigan.gov
Acknowledgements
Michelle Thorne, Kalamazoo County Health Dept.
Joel Blostein, Mich. Dept. Comm. Health
Questions?
Kyle Enger