Knight-Jones, Theodore J.D.

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      1 Mass vaccination, immunity and coverage:  Modelling population protection against foot-and-mouth disease in Turkish cattle  Theo Knight-Jones Epidemiologist, ILRI       2 FMD vaccine protection: - Requires several doses (≥3PD50) - Declines with time since vaccination Mass vaccination  Mass vaccination Population immunity ≈ Population vaccination history  [No. of doses, time since last dose] Population vaccine history ≈ Population age structure       3 Mass vaccination twice a year:   Population age-sex-vaccination distribution   Beef suckler cattle: Percentage of population       4 Mass vaccination twice a year:   Population age-sex-vaccination distribution   Beef fattener cattle: 9-10 doses 7-8 doses 5-6 doses Age Females 4-5 years  3-4 years  2-3 years  1-2 years  0-1 years Males Percentage of population Females Different production system = different age structure = different population immunity       5 Population vaccine history & immunity varies by region       6 Structure changes with births & deaths over the annual production cycle  New births = New unvaccinated animals Population immunity is constantly changing with population turnover & declining antibodies       7 Post-vaccination immunity also declines with time Vaccinated Spring+Autumn 2012 Vaccinated Autumn 2011+ Spring+Autumn 2012 Protection threshold once twice 3 times 2.3 2.4 2 0 Log10 (SP titre) Autumn mass vaccination Vaccinated  Autumn 2012 depending vaccine history       8 If 3 doses needed for “adequate” protection If multiple doses needed, variation in immunity resulting from variation in coverage becomes exaggerated District with 100% coverage: After 3 rounds: 100% of  cattle vaccinated 3 times District with 50% coverage: After 3 rounds: 50% x 50% x 50% = 12.5% vaccinated 3 times       9 Population coverage model • Describe population immunity over the production cycle with 2012 Turkish mass vaccination policy • Simulated the Turkish cattle population for each province • Age-structure by day and month of birth • Using data from national random surveys for each province and census data   • Dynamic population model representing the changing age structure for each province over the annual production cycle       10 Percentage never vaccinated 6 months after mass vaccination  – if eligible cattle always vaccinated Unvaccinated = Cattle too young at prior vaccination + New births since prior vaccination median values reported       11 Vaccinated ≥3 times  = Adult cattle Percentage vaccinated ≥3 times in lifetime  – if eligible cattle always vaccinated       12 Field studies and routine data found 40–99.9% vaccinated But not all eligible cattle will be vaccinated Results:  • Six months after the last round of vaccination almost half of the cattle aged ≤24 months remain unvaccinated Betapert distribution (minimum=40%, maximum=100%, most likely=80%)  • Only 50% of all cattle would have been vaccinated more than once with the last dose received ≤6 months ago       13 • Using regression models fitted to data from extensive post-vaccination sero-monitoring study [n=647] • Predict immunity for simulated population  LPBE SP titre = Time since vaccination + No. of times vaccinated From coverage to immunity       14 Population immunity predictions Primary course Autumn mass vaccination Two-dose primary course: Increases proportion of  6-12 month cattle above threshold by 25-40%  Log10 (SP titre) ≥2:    32% *25%-40%] Log10 (SP titre) ≥ 2: 27% [20%-35%] Log10 (SP titre) ≥ 2: 30% [24%-38%] Log10(SP titre) Threshold titre is useful but Titre ≈ Protection Antigenic similarity of: 1) Vaccine – 2) Test – 3) Field virus       15 District coverage and population immunity Modelled proportion vaccinated in a district at autumn vaccination against the percentage of cattle with a serotype O SP titre ≥1:102 in mid February       16 Sustained antibodies after single dose From: Selman P, Chénard G, Dekker A (2006) Cedivac-FMD; Duration of Immunity in cattle, sheep and pigs. Open session of the EuFMD, Paphos, Cyprus, 17-19 October 2006  Immunity reflects coverage Fewer problems        17 • Major immunity gaps despite biannual mass vaccination  • Improved vaccine required • ≥6PD50 vaccine now routine in Turkey • Two-dose primary course used in certain areas  • Immunity gaps will still exist • Each round of vaccination may exclude a quarter of all cattle • Often unavoidable  • Improved biosecurity measures required • Avoid over reliance on vaccine protection  Conclusions: Mass vaccination in Turkey 2012       18 

Mass vaccination, immunity and coverage: Modelling population protection against foot-and-mouth disease in Turkish cattle

https://cgspace.cgiar.org/bitstream/10568/52318/2/FMD%20in%20Turkish%20cattle.pdf

Mass vaccination, immunity and coverage: Modelling population protection against foot-and-mouth disease in Turkish cattle

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