There are two main challenges in assessing the severity of clinical outcomes during an epidemic of a newly emerging infection such as COVID-19. The first one ―surveillance―, is typically biased towards detecting clinically severe cases, particularly at the start of an epidemic when diagnostic capacity is limited (estimates of the proportion of fatal cases), the Case Fatality Ratio (CFR) ―may thus be biased upwards until the extent of clinically milder diseaseis determined―. The second one ―asymptomatic cases―, means there can be a period of two to three weeks between a case developing symptoms, subsequently being detected and reported and observing the final clinical outcome ―during a growing epidemic the final clinical outcome of the majority of the reported cases is typically unknown, mainly because dividing the cumulative reported deaths by reported cases will underestimate the CFR among these cases early in an epidemic―.
In the absence of a COVID-19 vaccine, a new study by Imperial College London (UK) weassess the potential role of a number of public health measures ―so-called Non-Pharmaceutical Interventions (NPIs)―, aimed at reducing contact rates in the population and thereby reducing transmission of the virus.
In this study, authors point to that the effectiveness of any one intervention in isolation is likely to be limited, requiring multiple interventions to be combined to have a substantial impact on transmission.
In this way, two fundamental strategies are possible: mitigation, which focuses on slowing but not necessarily stopping epidemic spread ―reducing peak healthcare demand while protecting those most at risk of severe disease from infection―; and suppression, which aims to reverse epidemic growth, reducingcase numbers to low levels and maintaining that situation indefinitely. Each policy has major challenges.
Hence, authors found that that optimal mitigation policies (combining home isolation of suspect cases, home quarantine of those living in the same household as suspect cases, and social distancing of the elderly and others at most risk of severe disease) might reduce peak healthcare demand by 2/3 and deaths by half.
However, the resulting mitigated epidemic would still likely result in hundreds of thousands of deaths and health systems ―most notably Intensive Care Units (ICUs)― being overwhelmed many times over. For countries able to achieve it, this leaves suppression as the preferred policy option. The study shows that in the UK and US context, suppression will minimally require a combination of social distancing of the entire population, home isolation of cases and household quarantine of their family members. This may need to be supplemented by school and university closures, though it should be recognised that such closures may have negative impacts on health systems due to increased absenteeism.
However, the major challenge of suppression is that this type of intensive intervention package ―or something equivalently effective at reducing transmission― will need to be maintained until a vaccine becomes available (potentially 18 months or more), given that we predict that transmission will quickly rebound if interventions are relaxed.
Authors also show that intermittent social distancing ―triggered by trends in disease surveillance― may allow interventions to be relaxed temporarily in relative short time windows, but measures will need to be reintroduced if or when case numbers rebound ―as long as there is only one person in the world with the virus, outbreaks will continue to be triggered uncontrolled unless strict controls and isolation measures are put in place to contain them―. In this way, the researchers conclude that social withdrawal and the closure of schools should occur approximately two-thirds of the time, that is, that out of every three months, only one could go out and still, in that month, the life should never be the way it was until then.
Last, while experience in China and now South Korea show that suppression is possible in the short term, it remains to be seen whether it is possible long-term, and whether the social and economic costs of the interventions adopted thus far can be reduced.
Link to the paper: https://doi.org/10.25561/77482
Editorial Disclaimer: information published during the 2020 COVID-19 pandemic may be updated frequently to reflect the dynamic nature of current understanding.