The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) was first reported as the cause of a rapidly spreading severe respiratory illness in Wuhan, China, towards the end of 2019. Since then, it has ravaged the entire world and emerged in many different variants.
In an attempt to prevent the spread of the coronavirus disease 2019 (COVID-19), most countries closed their borders to travelers entering or returning from outside or enforced lengthy and strict quarantines on those entering. The effectiveness of these policies will depend on the virus biology, including the specific variant present, the screening test used, and the degree to which travelers observe quarantine regulations.
A new study, released as a preprint on the medRxiv* server, examines the relative effectiveness of varying strategies, such as pre-flight testing, longer or shorter periods of quarantine, tests for release, or daily testing after the flight, using modeling tools. The results show that it is feasible to avoid quarantine by the use of lateral flow testing daily for five days, achieving higher reductions in the number of secondary cases caused by an infected traveler in the country of destination.
This conclusion is especially important now that several variants of concern (VOCs) have emerged and are spreading across international boundaries, posing a threat to the precarious control achieved by widespread vaccine rollouts in many developed countries.
What did the study do?
The researchers estimated the fraction of detected infections among the travelers that would be detected by each of these strategies, and the resulting decrease in secondary cases per infected traveler. When compared to the total volume of travelers entering each country, and the prevalence of the infection in the country of origin of the travelers, they were able to arrive at the risk of importation.
What did they find?
The scientists assume that about 28% of travelers actually follow quarantine guidelines, while 71% and 86% self-isolate when symptomatic or tested positive, respectively. Almost half of these will have an asymptomatic infection, with a fifth becoming symptomatic on arrival, and 7% symptomatic on the flight.
In this scenario, without either testing before the flight or quarantining after arrival, about two-thirds of entering infections will go undetected. Self-isolation of symptomatic travelers will reduce secondary cases by 45%.
14-day quarantine effective at reducing risk
With longer periods of quarantine, symptomatic patients will be more likely to be separated from the community at large at symptom onset, while those leaving quarantine will be mostly non-infectious at that point. In this setting, the utility of the post-quarantine test is reduced.
At 14 days of quarantine with a concluding test, the secondary case rate would be almost halved compared to self-isolation of symptomatic cases alone.
Should quarantine be shortened?
The findings also show that quarantine may be reduced to 10 days without a test to release, averting half the cases. A five-day quarantine with testing to release (if negative) at the end would produce similar reductions.
However, since many people will not follow quarantine regulations, almost 40% of infections will go undetected and cause transmission. To correct this, initial pre-flight testing (LFT or PCR) could reduce secondary cases by 91% to 95%. Alternatively, quarantine of all travelers in a supervised facility without testing to release could reduce secondary cases by 95%.
The most effective strategies thus include:
- Complete adherence to quarantine for 14 days without a test – zero secondary cases
- Quarantine with PCR on day 5 or LFT on day 7, to release into the community
- A pre-flight test and 5 days of quarantine with a test to release - 85% reduction with a pre-flight LFT/post-quarantine LFT or PCR, compared to 93% with a pre-flight PCR/post-flight LFT or PCR combination. The more rapid the test, the closer to actual take-off it can be, maximizing case detection.
- A daily LFT for five days could allow removal of mandatory quarantine, allowing only 7% of cases to go undetected and zero with ten days of testing.
What are the implications?
The choice of strategy will depend on the rate of importation and the incidence in the country of destination. Thus, the USA, with a high baseline incidence, may not be able to prevent imported infections by restricting travelers. Conversely, Singapore, with a low incidence but high volume of arriving travelers, would reduce its risk to 43%, but not less, with daily testing.
“Importation as a percentage of domestic incidence is therefore dependent on the epidemic at both ends of the travel route and the proportion of the variant of concern.” Both these factors are important in the context of the increased transmissibility, severity of illness, and immune-evasion capabilities, of the VOCs.
It then becomes necessary not just to estimate how many infected travelers are arriving, but how many secondary cases may occur if a given VOC goes undetected.
The LFT stands out as a reasonable choice in this setting, especially if used pre-flight as well as daily for 5-7 days, as it yields rapid results and thus prevents currently infectious travelers from boarding. The difficulties of quarantine are also significant, and adherence has been reported to be low, though higher with a positive test.
Thus, shorter quarantines may be more efficient at preventing viral spread in the real world, especially in managed facilities, and where most new cases are imported. Improving support to those in quarantine is an important way to improve adherence, as well.
medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.