Skip to 0 minutes and 7 seconds HILARY BOWER: Hello, everyone. In this session, we’ll review some of the knowledge gaps and explore what’s needed to better understand the spread of the COVID-19 epidemic. You’ll remember from Week 1 that the spread of an infectious disease is described by the R0, the basic reproductive number that describes the average number of people each case will infect if everyone is susceptible. And that R0 depends on three factors shown here in this slide. To properly assess these factors for COVID-19, we need certain critical pieces of information– take the duration of infectiousness. In an ideal world, infectiousness would start with symptoms, that way there is a visible alert that someone is sick. Unfortunately, this is not always the case.
Skip to 0 minutes and 57 seconds So critical things to know are– is it possible to be infected with SARS-CoV-2 but present no symptoms? And, if so, can a person without symptoms infect another person? We also need to know whether SARS-CoV-2 infectiousness ends with recovery. As described in Week 1, evidence suggests that primary transmission is by respiratory droplet but other coronaviruses have been known to aerosolize, creating a fine mist that hangs in the air and causes longer unseen exposure. And we need to know if this also applies to SARS-CoV-2. There is also a possibility that SARS-CoV-2 virus might transmit via other body products. Researchers recently found virus genetic material in stool samples.
Skip to 1 minute and 49 seconds While during the 2003 SARS epidemic, transmissible virus was found in respiratory droplets, stool, and urine from two to four weeks after the start of symptoms. We also need to know how long SARS-CoV-2 can survive outside the body. Work with other coronaviruses shows that droplets landing on surfaces, toys, tables, and suchlike can retain virus for up to a few days depending on temperature, humidity, and light and can be important preventable transmission routes. Fortunately, coronaviruses are sensitive to heat so normal cooking temperatures and good food hygiene should prevent transmission through food. Another important parameter in spread is who is still at risk and are they exposed to the virus?
Skip to 2 minutes and 39 seconds In a location without cases, the number of people still susceptible to infection will be large. Where there have already been cases, there may be many who are already infected but mildly and so not reported. Experience also suggests that people who have been exposed to other coronaviruses are immune for a period after illness, but how strong and how long that natural immunity is with SARS-CoV-2 is unknown and will underpin the risk of any second wave outbreaks. For this, sero- epidemiological studies, which identify infection from the presence of antibodies in the blood, are needed. Transmission works differently in different settings and among different groups due to varying patterns of contact and risk factors.
Skip to 3 minutes and 29 seconds Understanding transmission in settings such as households and health care facilities where the number of individuals are known and among more vulnerable groups is important to inform intervention in different contexts. Answering questions such as “how health care workers need to be protected” and “why in the first large analysis from China young children aged 0 to 9 years make up less than 1% of reported cases” can guide response as can assessing the impact of risk factors such as comorbidities and pregnancy. Whether SARS-CoV-2 superspreading events are a phenomenon, for example, of delayed diagnosis co-infection or particular contact patterns is also an important unknown.
Skip to 4 minutes and 19 seconds Particular efforts are needed to understand the risks in populations such as in parts of sub-Saharan Africa where limited health systems can mean limited surveillance and delayed detection and response. A key challenge to measuring the spread of SARS-CoV-2 is that case definitions focus on people sick enough to seek care. This means people who stay at home and self-treat are missed, obscuring not only how far the virus has spread but also how much unreported cases might be contributing to transmission. Case definition also affects other statistics.
Skip to 4 minutes and 57 seconds Modelling suggests that the current case fatality rate for COVID-19 of 2.3% based on notified cases would reduce to below 1% if true infection rates were taken into account and essential parameters for calculating speed of transmission such as the serial interval, that’s the time between symptoms appearing in one person and the next, how long someone infected remains infectious, and the R0 will change when all infections are included. To uncover the true extent of SARS-CoV infection, population-based sero- prevalence surveys to detect antibodies are essential.
Skip to 5 minutes and 40 seconds In summary, to better understand the potential for the spread of SARS-CoV-2 in different geographies and context and in the most vulnerable populations worldwide, we need to determine the R0, establish the proportion of those infected that develop symptoms and seek care, find out the duration of infectiousness and its relationship to symptoms, investigate the main routes of transmission and see if there are any other rarer routes, establish the risk factors for infection and severe disease, and identify risk factors for superspreading events. All these areas of study plus the natural history and origin of SARS-CoV-2 are the focus of the WHO research blueprint, which researchers around the world are now striving to address.
How can we better understand spread?
Hilary Bower discusses what more we need to know about the spread of SARS CoV-2 causing COVID-19 (recorded 25th February 2020).
Intended learning outcomes
- Explain measures needed to better understand spread of COVID-19
- Appraise current problems with estimates, case definitions, fragile contexts
- Assess knowledge gaps and formulate way forward
© London School of Hygiene & Tropical Medicine 2020