Table 3. Incidence of pertussis in household contacts by characteristic of primary cases (n = 2,852).

Characteristic of primary case	Incidence of pertussis among contacts		Odds ratio	95% CI
	%	n/total
Sex
Female	16.0	245/1,528	1.0	0.8 to 1.2
Male	16.2	214/1,324	Reference
Age in years
< 1	8.9	60/671	Reference
1	9.6	10/104	1.1	0.5 to 2.2
2–3	10.9	25/229	1.2	0.7 to 2.0
4–6	14.4	47/326	1.7	1.1 to 2.6
7–10	15.3	91/595	1.8	1.3 to 2.6
11–18	14.9	54/363	1.8	1.2 to 2.6
19–40	31.0	90/290	4.6	3.2 to 6.6
> 40	29.9	82/274	4.3	3.0 to 6.3
Microbiological confirmation (PCR and/or culture)
Yes	10.7	219/2,055	Reference
No	27.0	61/226	3.1	2.2 to 4.3
Unknown	30.8	158/513	3.7	2.9 to 4.7
Hospitalisation
Yes	10.0	48/479	0.6	0.4 to 0.8
No	17.4	403/2,312	Reference
Number of contacts
≤ 2	19.5	44/226	Reference
3–4	16.0	181/1,133	0.8	0.6 to 1.1
> 4	15.7	234/1,493	0.8	0.6 to 1.1
Vaccination statusa
Fully vaccinated	14.1	188/1,331	Reference
Undervaccinated/ Unvaccinated/Unknown	17.8	271/1,521	1.3	1.1 to 1.6

CI: confidence interval.

^a Vaccination status was categorised as fully vaccinated (≥ 4 doses of vaccine), undervaccinated (< 4 doses) or unvaccinated (no dose).