TABLE 3.

Estimated Chlamydia Transmissions and Corresponding Absolute Reduction in Onward Transmission and Number Needed to Treat to Interrupt Transmission, Assuming No Partner Notification, at a Single Genitourinary Medicine Clinic: Great Britain, 1999–2004

	Men		Women
Partnership Type	< 25 Years	≥ 25 Years	< 25 Years	≥ 25 Years
Primary chlamydia transmissions from index patients
Live-in partnerships
No. of transmissions	6	24	22	18
Assumed probability of transmission, %	50	60	50	60
Total no. of partnerships	13	40	43	30
Regular partnerships
No. of transmissions	20	17	19	12
Assumed probability of transmission, %	40	48	40	48
Total no. of partnerships	50	35	47	25
Casual partnerships
No. of transmissions	31	18	10	8
Assumed probability of transmission, %	30	36	30	36
Total no. of partnerships	103	51	33	22
Primary transmissions,a total no.	58	59	50	38
All partnerships, total no.	167	126	123	78
Secondary chlamydia transmissions from index patients’ partners
Live-in partnerships
No. of transmissions	0	0	0	0
Assumed probability of transmission, %	50	60	50	60
Total no. of partnerships	0	0	0	0
Regular partnerships
No. of transmissions	20	17	38	12
Assumed probability of transmission, %	40	48	40	48
Total no. of partnerships	50	35	94	25
Casual partnerships
No. of transmissions	93	37	39	24
Assumed probability of transmission, %	30	36	30	36
Total no. of partnerships	310	102	131	67
Secondary transmissions,a total no.	113	53	77	36
All partnerships, total no.	361	137	225	93
AROT and NNTITb
Live-in partnerships
AROT	0	0	0	0
NNTIT	∞	∞	∞	∞
Regular partnerships
AROT	0.40	0.48	0.80	0.48
NNTIT	2.50	2.08	1.25	2.08
Casual partnerships
AROT	0.90	0.72	1.20	1.08
NNTIT	1.11	1.39	0.83	0.93
All partnerships
AROT	0.68	0.42	0.63	0.47
NNTIT	1.47	2.35	1.60	2.14

Note. AROT = absolute reduction in onward transmission; NNTIT = number needed to treat to interrupt transmission.

Source. Data from the Second National Survey of Sexual Attitudes and Lifestyles were used with data from the genitourinary medicine clinic to calculate estimates.

^aBecause of rounding, totals may differ slightly from the sum of the numbers expressed to 0 decimal places.

^bWe calculated the estimated no. of transmissions assuming the partner numbers shown in Appendices 2 and 3 for index patients and index patients’ partners, respectively. The absolute risk of transmission is calculated as the no. of secondary transmissions divided by the no. of index cases’ partners. The NNTIT is calculated as the reciprocal of the AROT.