Serial interval

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The serial interval, in the epidemiology of communicable (infectious) diseases, refers to the time between successive cases in a chain of transmission.[1]

The serial interval is generally estimated from the interval between clinical onsets, in which case it is the 'clinical onset serial interval' when these quantities are observable. It could, in principle, be estimated by the time interval between infection and subsequent transmission. If the timing of infection transmission in deference to a person's clinical onset is TA, and the incubation period of a subsequent case is IB, then the clinical onset serial interval is calculated by TA + IB. Real data best reflect a frequency distribution when observing a single primary case and its associated secondary cases.[2] If the timing of transmission events is not skewed during the infectious period, then the average serial interval is the sum of the average latent period (from infection to infectiousness) and half the average infectious period.

Serial intervals can vary widely, especially for some lifelong diseases (HIV infection, Chickenpox, Herpes). The serial interval for SARS was 7 days.[3] For COVID-19, a review of the published literature shows serial interval estimates to be 4-8 days.[4]

Related but distinct quantities include : the 'average transmission interval' sum of average latent and infectious period; the 'incubation period' between infection and disease onset; the 'latent period' between infection and infectiousness.


  1. ^ Last J.M. (2001) A Dictionary of Epidemiology, Oxford University Press.
  2. ^ Fine, P. E. M. (2003). "The Interval between Successive Cases of an Infectious Disease". American Journal of Epidemiology. 158 (11): 1039–1047. doi:10.1093/aje/kwg251. PMID 14630599.
  3. ^ Lipsitch, M.; Cohen, T.; Cooper, B.; Robins, J. M.; Ma, S.; James, L.; Gopalakrishna, G.; Chew, S. K.; Tan, C. C.; Samore, M. H.; Fisman, D.; Murray, M. (2003). "Transmission Dynamics and Control of Severe Acute Respiratory Syndrome" (PDF). Science. 300 (5627): 1966–1970. doi:10.1126/science.1086616. PMC 2760158. PMID 12766207.
  4. ^ Park, Minah; Cook, Alex R.; Lim, Jue Tao; Sun, Yinxiaohe; Dickens, Borame L. (2020-03-31). "A Systematic Review of COVID-19 Epidemiology Based on Current Evidence". Journal of Clinical Medicine. 9 (4): 967. doi:10.3390/jcm9040967. ISSN 2077-0383. PMC 7231098. PMID 32244365.