Case isolation, contact tracing, and physical distancing are pillars of COVID-19 pandemic control, not optional choices
Authors: Chandini Raina MacIntyre
Journal: The Lancet Infectious Diseases (2020)

Amid the urgency of the COVID-19 pandemic response, the study by Adam Kucharski and colleagues in The Lancet Infectious Diseases1Kucharski AJ Klepac P Conlan AJK et al.Effectiveness of isolation, testing, contact tracing, and physical distancing on reducing transmission of SARS-CoV-2 in different settings: a mathematical modelling study.Lancet Infect Dis. 2020; (published online June 16.)https://doi.org/10.1016/S1473-3099(20)30457-6Summary Full Text Full Text PDF Scopus (498) Google Scholar reminds us that all available control measures should be used together to drive the effective reproduction number (Reff) below 1. The word epidemic is widely misused, but true epidemic respiratory infections (when R >1) grow exponentially within days or weeks and thus we do not have time on our side. Delays or mistakes in disease control can be costly. Using UK contact data from 40 162 UK participants of the BBC Pandemic study and optimistic assumptions, Kucharski and colleagues modelled individual-level transmission during the COVID-19 pandemic stratified by setting (household, work, school, or other) with and without various non-pharmaceutical interventions (NPIs) including different testing, isolation, tracing, and physical distancing scenarios. Their findings showed that we do not have the luxury of picking and choosing between available NPIs to control COVID-19. A high proportion of cases need to be identified and isolated (in hospital or at home), with a high rate of contact tracing and quarantine along with physical distancing, to achieve and maintain epidemic control. This finding might sound obvious, but on Feb 18, 2020, the eighth meeting of the Scientific Advisory Group for Emergencies in the UK concluded that "when there is sustained transmission in the UK, contact tracing will no longer be useful".2Scientific Advisory Group for EmergenciesAddendum to the eighth SAGE meeting on Covid-19, 18 February 2020 Held in 10 Victoria Street, London, SW1H 0NN.https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/888776/S0376_Eighth_SAGE_meeting_on_Wuhan_Coronavirus__Covid-19__.pdfDate: 2020Date accessed: June 1, 2020Google Scholar In Kucharski and colleagues' study, combinations of case isolation with contact tracing reduced transmission by 47% (self-isolation combined with app-based tracing) to 64% (self-isolation combined with manual contact tracing of all contacts) compared with mass testing and case isolation alone, which reduced transmission by 2% (mass random testing of 5% of the population each week) to 37% (self-isolation plus household quarantine). The COVID-19 pandemic is an unprecedented event in our lifetimes, and we have enjoyed the luxury of vaccines to control serious epidemic infections in the past century. As a result, we have rarely needed some of the other pillars of epidemic control on a mass scale. In the absence of a vaccine, control of COVID-19 relies on four main strategies. First, identifying new cases by surveillance and testing and isolating them to prevent further transmission. Second, tracking all contacts of cases and quarantining them for the incubation period, to reduce the risk of further transmission; quarantine is especially crucial for infections with asymptomatic or pre-symptomatic transmission,3Peak CM Childs LM Grad YH Buckee CO Comparing nonpharmaceutical interventions for containing emerging epidemics.Proc Natl Acad Sci USA. 2017; 114: 4023-4028Crossref PubMed Scopus (146) Google Scholar and retrospective contact tracing to identify the source of infection of each case is also essential. Third, social distancing, ranging from spatial separation of 1–2m2 to banning of mass gatherings and imposing lockdowns. And fourth, travel restrictions to stop the importation of infections.5Costantino V Heslop DJ MacIntyre CR The effectiveness of full and partial travel bans against COVID-19 spread in Australia for travellers from China.J Travel Med. 2020; (published online May 22.)DOI:10.1093/jtm/taaa081Crossref PubMed Scopus (10) Google Scholar Additionally, the use of face masks of even moderate effectiveness might help keep the Reff lower than 1.6MacIntyre CR Wang Q Physical distancing, face masks, and eye protection for prevention of COVID-19.Lancet. 2020; (published online June 1.)https://doi.org/10.1016/S0140-6736(20)31183-1Summary Full Text Full Text PDF PubMed Scopus (97) Google Scholar The best available evidence suggests that masks are effective against beta coronaviruses in community settings and should be used in combination with physical distancing of at least 1 m.4Chu DK Akl EA Duda S et al.Physical distancing, face masks, and eye protection to prevent person-to-person transmission of SARS-CoV-2 and COVID-19: a systematic review and meta-analysis.Lancet. 2020; (published online June 1.)https://doi.org/10.1016/S0140-6736(20)31142-9Summary Full Text Full Text PDF Scopus (2489) Google Scholar Kucharski and colleagues showed that the first, second, and third strategies are not competing choices: none alone are enough, all are highly influential in epidemic control, and should be combined to bring Reff below 1. The lowest Reff in Kucharski and colleagues' study was achieved with a combination of case finding and isolation, contact tracing, quarantine, and social distancing (limiting outside contacts to four per day). For case finding, the authors showed that random testing (undertaken in some countries) barely reduced Reff and was not an efficient use of resources. Targeted testing of high-risk contacts in outbreak settings, regardless of symptoms, is much more efficient, especially given that asymptomatic infections are common.7Roxby AC Greninger AL Hatfield KM et al.Outbreak investigation of COVID-19 among residents and staff of an independent and assisted living community for older adults in Seattle, Washington.JAMA Intern Med. 2020; (published online May 21.)DOI:10.1001/jamainternmed.2020.2233Crossref PubMed Scopus (88) Google Scholar Countries that are doing random testing but not testing asymptomatic contacts should review this strategy. For contact tracing, Kucharski and colleagues looked at manual contact tracing with self-isolation, which reduced transmission by 57%, and app-based tracing with 53% uptake, which improved that somewhat when in combination with manual tracing. However, the real value of app-based tracing is when case numbers are high and human resources stretched. Each case requires ten to 25 contacts to be traced, so the human resources requirement for manual tracing can easily be exceeded in a large epidemic. In that case, the use of an app might become essential for epidemic control.8Ferretti L Wymant C Kendall M et al.Quantifying SARS-CoV-2 transmission suggests epidemic control with digital contact tracing.Science. 2020; 368eabb6936Crossref PubMed Scopus (1535) Google Scholar Without contact tracing, isolation alone will not reduce Reff below 1 and will result in a blow-out of the epidemic. The challenge of COVID-19, by contrast with severe acute respiratory syndrome, is that substantial transmission can occur in the absence of symptoms,9He X Lau EHY Wu P et al.Temporal dynamics in viral shedding and transmissibility of COVID-19.Nat Med. 2020; 26: 672-675Crossref PubMed Scopus (2719) Google Scholar and accumulating evidence exists of aerosol transmission of the virus.10Fears AC Klimstra WB Duprex P et al.Comparative dynamic aerosol efficiencies of three emergent coronaviruses and the unusual persistence of SARS-CoV-2 in aerosol suspensions.medRxiv. 2020; (published online April 18.) (preprint).DOI: 10.1101/2020.04.13.20063784PubMed Google Scholar Although Kucharski and colleagues' study is specific for the UK, the findings are relevant to all countries. For countries that are opening up for business and resuming social activities, or where protests are occurring, as the number of social contacts increase, NPIs become even more crucial. It might even be worthwhile for countries to invest in strategies to vastly improve the uptake of contact-tracing apps to enable rapid response to resurgence of COVID-19. If you do not trace, you leave a chain of transmission free to grow undetected and exponentially. With 80% of cases being mild, it might take several generations of silent epidemic growth before a new outbreak is even recognised. I report grants from Sanofi, outside the submitted work. Effectiveness of isolation, testing, contact tracing, and physical distancing on reducing transmission of SARS-CoV-2 in different settings: a mathematical modelling studyConsistent with previous modelling studies and country-specific COVID-19 responses to date, our analysis estimated that a high proportion of cases would need to self-isolate and a high proportion of their contacts to be successfully traced to ensure an effective reproduction number lower than 1 in the absence of other measures. If combined with moderate physical distancing measures, self-isolation and contact tracing would be more likely to achieve control of severe acute respiratory syndrome coronavirus 2 transmission. Full-Text PDF Open Access