I've read that the incubation period of the covid-19 virus ranges from 2 to 14 days with rare exceptions (e.g. here). However, a person carrying a virus could theoretically be contagious and transmit it without themselves having developed symptoms. My questions:

  1. Can carriers of covid-19 infect others before developing noticeable symptoms themselves?
  2. If so, what's known about the duration from the time of infection to the time the carrier becomes contagious?

4 Answers 4


To question 1, infection of others before symptoms of the carrier

Yes, carriers of Covid-19 can infect others before developing noticeable symptoms themselves.

There is a study to a very traceable transmission chain in a company in Germany. Person 1 was infected from a from China comming person 0 after January 19, 2020.

  • A person 3 had contact only with person 1 on January 20 and 21. However, person 1 had first symptoms on the January 24th, so the infection occurred 3 days before symptoms of the carrier.

  • Another person 4 had also contact only with person 1 from January 21 to January 24. This infection occurred within 0 to 3 days before person 1 first symptoms.

This study can be read in detail here: https://www.nejm.org/doi/10.1056/NEJMc2001468.

There exist also an interview with person 1, which states that he met person 0 on Monday (January 20).

  • Person 0 doesn't show symptoms of illness during their presence. Symptoms were established on their return trip on January 22, so person 1 was infected 1 to 2 days before symptoms of person 0.

The interview is readable here: https://www.br.de/radio/bayern1/coronavirus-in-bayern-100.html.

To question 2, duration until the carrier becomes contagious

  • The above study shows that person 1 infected person 3 already after 1 to 2 days after infection themselves.

In addition, there is an study to 312 infections in Austria about the serial interval which refers to the time between successive cases of transmission.

  • According to this study, carriers of the virus contribute significantly to an infection already after day 1. The risk of transmission is highest on day 3 and after 10 days the risk of transmission drops significantly, but is still present.

You can see this study here: https://www.ages.at/en/wissen-aktuell/publikationen/schaetzung-des-seriellen-intervalles-von-covid19-oesterreich


Haaretz reports (Hebrew, possible paywall) about two studies, in Singapore and in Tianjin, China, on this matter:

  • The average "generation gap" (i.e. the duration OP asked about - from contraction to becoming contagious) was established as 5.2 days (Singapore data) or 3.9 days (Tianjin data).
  • The fraction of contractions due to infection by asymptomatic patients (whether they later become symptomatic or not) is 45%-84% (Singapore data) or 5%-87% (Tianjin data).

Bottom line answers (based on these studies):

  1. Yes, they can and do infect others.
  2. Carriers become contagious a few days after contracting the virus (probably 4 or 5).

There's some computer modelling to suggest that 1:10 infections are caught from people with asymptomatic infection, and the virus is thought to be exhaled as the person breathes out. In fact a 9 patient study showed peak virus shedding is at about 5 days just before the patient becomes symptomatic when their immune system ramps up making antibodies to counter the virus, and reduce viral shedding.

Uptodate ( not firewalled at present)

Transmission of SARS-CoV-2 from asymptomatic individuals (or individuals within the incubation period) has also been described [17-21]. However, the extent to which this occurs remains unknown. Large-scale serologic screening may be able to provide a better sense of the scope of asymptomatic infections and inform epidemiologic analysis; several serologic tests for SARS-CoV-2 are under development [22].



The clinical courses in subjects under study were mild, all being young- to middle-aged professionals without significant underlying disease. Apart from one patient, all cases were first tested when symptoms were still mild or in the prodromal stage, a period in which most patients would present once there is general awareness of a circulating pandemic disease5 . Diagnostic testing suggests that simple throat swabs will provide sufficient sensitivity at this stage of infection. This is in stark contrast to SARS. For instance, only 38 of 98 nasal or nasopharyngeal swab samples tested positive by RT-PCR in SARS patients in Hong Kong15. Also, viral load differed considerably. In SARS, it took 7 to 10 days after onset until peak RNA concentrations (of up to 5x105 copies per swab) were reached13,14. In the present study, peak concentrations were reached before day 5, and were more than 1000 times higher. Successful live virus isolation from throat swabs is another striking difference from SARS, for which such isolation was rarely successful16-18. Altogether, this suggests active virus replication in upper respiratory tract tissues, where only minimal ACE-2 expression is found and SARS-CoV is therefore not thought to replicate19. At the same time, the concurrent use of ACE-2 as a receptor by SARS-CoV and SARS-CoV-2 corresponds to a highly similar excretion kinetic in sputum, with active replication in the lung. SARS-CoV was found in sputum at mean concentrations of 1.2-2.8x106 copies per mL, which corresponds to observations made here13.


  • Wait, if "peak shedding" is just before the onset of symptoms, shouldn't there be more than 1 in 10 infections caused by asymptomatics?
    – einpoklum
    Mar 17, 2020 at 9:49
  • They're not shedding it significantly into the environment i.e. coughing Mar 17, 2020 at 10:22
  • Added the link and extract for you Mar 17, 2020 at 10:34

re 1: maybe

re 2: Incubation period is about 4.5 to 5.8 days

but we don't know for sure, the virus is too new, and already there are possibly two strains (newer strain from Europe and USA is possibly more infectious), not sure if these possible two strains are handled differently, but some researchers are worried.

One problem with so many infected people is that it gives the virus to chance mutate more.


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