Serial interval

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The serial interval (engl. Serial interval ) referred to in the epidemiology of infectious diseases of a disease in two consecutive members of a time interval between the analog phase infection chain , ie from generation after generation n n + 1. The symptoms of an infectious disease are usually easier to date than the infection or the onset of infectivity, for example. Therefore, the serial interval is usually determined according to the symptoms, for example as the time interval between the onset of the symptoms (clinical "illness") of a person after the incubation period and the onset of symptoms in a person infected by them in the next generation of the chain of infection. The determination is made by observing a chain of infection, if possible under controlled conditions, for example in a hospital. In such cases, the serial interval measures the time between the onset of the disease in successive generations of infected people , assuming mean incubation times .

By contrast, generation time (Engl. Generation time ) a term that refers to the time between infection of a person and the time of secondary infection one of her infected person. However, the time of infection can usually not be determined exactly, so that the serial interval, which is easier to determine, is used as an approximation for the generation time.

The length of the serial interval differs from the incubation time - the time from infection to the onset of the disease (appearance of the first symptoms). The person in question may be able to pass the pathogen on, even if no symptoms of the disease can yet be identified. Then the latency period is shorter than the incubation period. This is often the case with COVID-19 , for example . Likewise, an infected person can no longer be infectious after a while, even if symptoms of the disease are still present. In general, the serial interval is longer than the incubation time.

The minimum of the observed serial intervals gives an indication of how long the pre-infectious phase is, i.e. the time between infection and the onset of infectivity. This can indirectly determine when an infected person is contagious.

The serial interval has very different lengths. For some chronic infectious diseases such as herpes and HIV, it is very large. For Covid-19 , the serial interval averages 7.5 days (standard deviation 3.4 days) according to a study with 425 patients from China (according to another study based on 28 cases, four days). A study with 468 confirmed pairs of infections from all over China in January / February 2020 also came to a serial interval of on average 3.96 (95 percent confidence interval 3.53 to 4.39 days, standard deviation 4.75 days). There were also 59 cases in which the symptoms appeared earlier in the infected person than in the infected person (12.6 percent of the cases). For SARS , it was 8.4 days (standard deviation 3.8) when evaluating cases from Singapore, but was higher (value of 10) for cases early in the epidemic when no strict measures were in place, so it was a total of 8 to 12 was estimated.

The comparison of the number of new infections over two successive generation times is used to estimate the number of reproductions of an epidemic. For the generation time or its approximation by the serial interval, a time distribution function is also used in epidemiology with more precise treatment and there are various relationships between Reproduction number R and growth rate r depending on the choice of the distribution function (see Euler-Lotka equation ). This particularly depicts the different infectiousness over the course of the infectious period. The generation time can also change over time if measures are taken, for example to block contact or isolation, because then not only does the number of subsequent infections decrease, but the generation time is also shortened, since in this case most infections occur at the beginning of the infection.

Individual evidence

  1. ↑ a b John Last (Ed.), A Dictionary of Epidemiology, IEA, Oxford UP 2001, p. 167
  2. ↑ a b Kenneth J. Rothman, Sander Greenland, Timothy Lash, Modern Epidemiology, 3rd edition, Lippincott 2008, p. 560. Assuming average incubation periods, the serial interval reflects the amount of time elapsed between exposure from one generation of spread to the next .
  3. ↑ Luca Ferretti et al., Quantifying SARS-CoV-2 transmission suggests epidemic control with digital contact tracing , Science, March 32, 2020
  4. ↑ a b c SARS-CoV-2 Profile on Coronavirus Disease-2019 (COVID-19) , Robert Koch Institute, March 23, 2020
  5. ↑ Qun Li et al. a .: Early Transmission Dynamics in Wuhan, China, of Novel Coronavirus – Infected Pneumonia Preprint , New England Journal of Medicine, January 29, 2020, accessed March 25, 2020
  6. ↑ Zhanwei Du, Lin Wang, Lauren Meyers and others. a .: The serial interval of COVID-19 from publicly reported confirmed cases], Preprint, CDC Emergent Effective Diseases Research Letter 2020, CDC, Abstract . Retrieved March 25, 2020
  7. ↑ M. Lipsitch, M. Murray et al: Transmission Dynamics and Control of Severe Acute Respiratory Syndrome , Science, Volume 300, 2003, pp. 1966-1970, PMID 12766207
  8. ↑ a b Estimate of the current development of the SARS-CoV-2 epidemic in Germany - Nowcasting , Epidemiological Bulletin 17/2020, Robert Koch Institute. April 23, 2020, evaluation of the reproduction factor of the Covid-19 epidemic in Germany by April 9.