How long people with covid-19 should self-isolate depends on the period for which they remain infectious. On 4 January, the US Centers for Disease Control (CDC) updated covid-19 isolation and quarantine recommendations with shorter isolation (for asymptomatic and mildly ill people) and quarantine periods of 5 days to focus on the period when a person is most infectious, followed by continued masking for an additional 5 days.1 This policy was based on a modelling study from the United Kingdom by Bays et al which showed that after the 5th day after a positive test, an estimated 31% of persons remained infectious.2 All the authors of this modelling study, which was published as a pre-print on 24/12/2021, work for UK Health Security Agency (UK HSA).
On 22 December 2021, the UK HSA reduced self-isolation for covid-19 cases in England from 10 to 7 days following negative lateral flow tests on days 6-7. The UK HSA stated that that a 7-day isolation period alongside 2 negative lateral flow tests had nearly the same protective effect as a 10-day isolation period without testing for people with covid-19.
On 1 January, the UK HSA published a blog on using lateral flow tests to reduce the self-isolation period.3 The blog provides background to explain the reasons for the difference between the policies. It also stated that after 10 days self-isolation, 5% of people will still be infectious; and that ending self-isolation after 7 days and two negative lateral flow tests resulted in a similar level of protection.
The two negative test results are essential in safely supporting the end of self-isolation. Without testing, modelling suggests that 16% of people would still be infectious after day 7. On 13 January, the Health Secretary Sajid Javid stated that from 17 January people will be able leave isolation from the start of day six after two negative lateral flow tests on days 5-6.
Both the US CDC and the UK HSA have based their length of isolation policy mainly on a single modelling study. The data on which the modelling was based It is therefore very important. Bays et al provide a single reference for “infectious period distribution”, a UKHSA modelling paper by Birrell et al published on 31 May 2021.4 Hence, it did not contain any information about the Omicron variant of SARS-CoV-2. It gives as a data source: “the Wuhan outbreak additionally provides information on epidemiological parameters: the duration of infectiousness, the mean time from infection to symptom onset; the probability of dying given infection and the mean time from symptoms onset to death”.
The Wuhan report by Li et al was published in New England Journal of Medicine on 26 March 2020.5 It does not contain any empirical information on the time for which cases were infectious. It only estimates the mean serial interval (MSI), based on six cases only, which represents the average time between the time of symptom onset of a primary case and that of a secondary case.6 The MSI is widely used in infectious disease surveillance and control because it allows investigators to identify epidemiologic links between cases and to diagnose new cases that have such epidemiologic links with laboratory-confirmed cases. The MSI in Li et al is 7.5±3.4 days (95% CI, 5.3 to 19). There is no information specifically about infectious periods.
Policies in both the UK and US are based on limited data and only on the wild-type SARS-CoV2 variant. Ideally, there should be population-based studies which included daily monitoring of culturable Omicron variant viral shedding (or even better actual transmission, which should be available from large databases) and PCR and lateral flow testing. A 2020 (so pre-Delta) rapid scoping review and analysis from Ireland of available evidence for serial testing asymptomatic and symptomatic cases showed substantial variation in the estimates, and how the infectious period was inferred.7 One study provided an approximate median infectious period for asymptomatic cases of 6.5–9.5 days. Median pre-symptomatic infectious period across studies varied over <1–4 days (and there are several recent studies of the Omicron incubation period showing it is short). Estimated mean time from symptom onset to two negative RT-PCR tests was 13.4 days (95% CI 10.9 to 15.8), but was shorter when studies included children or less severe cases. The only currently available study of the Omicron variant is a small Japanese report which showed the number and percentage of Omicron variant virus isolation positive samples as 7/17 (41.2%) after three to six days and 2/18 (11.1%) at seven to nine days.10
The change in isolation policy for people with covid-19 in England is a pragmatic step that will allow people to return to productive work, education and social activities more quickly. People may also be more likely to comply with a shorter isolation period. But the changes should have been based on careful monitoring and review based on new data on the Omicron variant, not on data on the wild type of SARS-CoV-2. We therefore need careful evaluation of the new shorter isolation period to ensure that people are following the guidance on self-testing and symptoms, and not ending their isolation period too early, and thereby putting others at risk of infection from covid-19.
A version of this article was first published in the British Medical Journal.
- US Centers for Disease Control. What We Know About Quarantine and Isolation: Why CDC Shortened Isolation and Quarantine for the General Population: US Centers for Disease Control, 2022.
- Bays D, Whiteley T, Pindar M, et al. Mitigating isolation: The use of rapid antigen testing to reduce the impact of self-isolation periods.medRxiv2021:2021.12.23.21268326. doi: 10.1101/2021.12.23.21268326
- UK Health Security Agency. Using lateral flow tests to reduce the self-isolation period: UK Health Security Agency, 2022.
- Birrell P, Blake J, van Leeuwen E, et al. Real-time nowcasting and forecasting of COVID-19 dynamics in England: the first wave.Philosophical Transactions of the Royal Society B: Biological Sciences2021;376(1829):20200279. doi: doi:10.1098/rstb.2020.0279
- Li Q, Guan X, Wu P, et al. Early Transmission Dynamics in Wuhan, China, of Novel Coronavirus–Infected Pneumonia.New England Journal of Medicine2020;382(13):1199-207. doi: 10.1056/NEJMoa2001316
- Vink MA, Bootsma MCJ, Wallinga J. Serial Intervals of Respiratory Infectious Diseases: A Systematic Review and Analysis.American Journal of Epidemiology2014;180(9):865-75. doi: 10.1093/aje/kwu209
- Byrne AW, McEvoy D, Collins AB, et al. Inferred duration of infectious period of SARS-CoV-2: rapid scoping review and analysis of available evidence for asymptomatic and symptomatic COVID-19 cases.BMJ Open2020;10(8):e039856. doi: 10.1136/bmjopen-2020-039856
- van Kampen JJA, van de Vijver DAMC, Fraaij PLA, et al. Duration and key determinants of infectious virus shedding in hospitalized patients with coronavirus disease-2019 (COVID-19).Nature Communications2021;12(1):267. doi: 10.1038/s41467-020-20568-4
- Monel B, Planas D, Grzelak L, et al. Release of infectious virus and cytokines in nasopharyngeal swabs from individuals infected with non-B.1.1.7 or B.1.1.7 SARS-CoV-2 variants.medRxiv2021:2021.05.20.21257393. doi: 10.1101/2021.05.20.21257393
- National Institute of Infectious Diseases Disease Control and Prevention Center. Active epidemiological investigation on SARS-CoV-2 infection caused by Omicron variant (Pango lineage B.1.1.529) in Japan: preliminary report on infectious period: National Institute of Infectious Diseases Disease Control and Prevention Center, National Center for Global Health and Medicine, 2022. https://www.niid.go.jp/niid/en/2019-ncov-e/10884-covid19-66-en.html