Fig. 1

Reported contact persons and recruitment trees. a The empirical reversed cumulative distribution of degree (number of contact persons per participant) is indicated with black circles. The line is the fitted theoretical Poisson inverse-Gaussian distribution with mean μ: 19.6 (95 % CI 18.3–21.1) and dispersion parameter λ: 2.0 (95 % CI 1.8–2.1). b Number of participants (nodes) per recruitment tree. Most recruitment ‘trees’ only consisted of one participant (the seed), two trees consisted of 11 participants. c Number of waves that recruitment trees reached by peer recruitment, with seeds in wave 0. One recruitment tree reached 6 waves of recruits. d Recruitment generation interval. Red line indicates median generation interval