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.
DOI: https://doi.org/10.1136/bmj.o184
References
1. 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.
2. Bays D, Whiteley T, Pindar M, et al. Mitigating isolation: The
use of rapid antigen testing to reduce the impact of self-isolation periods. medRxiv 2021:2021.12.23.21268326. doi:
10.1101/2021.12.23.21268326
3. UK Health Security Agency. Using lateral flow tests to reduce the
self-isolation period: UK Health Security Agency, 2022.
4. 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 Sciences 2021;376(1829):20200279. doi:
doi:10.1098/rstb.2020.0279
5. Li Q, Guan X, Wu P, et al. Early Transmission Dynamics in Wuhan,
China, of Novel Coronavirus–Infected Pneumonia. New England Journal of Medicine 2020;382(13):1199-207. doi:
10.1056/NEJMoa2001316
6. Vink MA, Bootsma MCJ, Wallinga J. Serial Intervals of Respiratory
Infectious Diseases: A Systematic Review and Analysis. American Journal of Epidemiology 2014;180(9):865-75. doi:
10.1093/aje/kwu209
7. 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 Open 2020;10(8):e039856. doi: 10.1136/bmjopen-2020-039856
8. 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
Communications 2021;12(1):267. doi: 10.1038/s41467-020-20568-4
9. 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. medRxiv
2021:2021.05.20.21257393. doi: 10.1101/2021.05.20.21257393
10. 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
Comments