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)

Td and Tdap Td boosts tetanus and diphtheria immunity (given to 11-12 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

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.

Td & Tdap by month, 97-2007 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 2006. 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.

Td & Tdap by year, 97-2007 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.

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 2006. 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 11-12 year olds increased every year from 2003 to 2006.

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 15-18 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.

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

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.

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 2006. 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.

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. 2006 in MI.

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 (4000 17y vs. 8700 18y), and because students are entering Washtenaw from out of state.

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’.

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.

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.

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