This is the seventeenth document in a series of discussion notes addressing fundamental questions about the COVID-19 crisis and response strategies. These documents are based on the best scientific information available and may be updated as new information comes to light
Written by Clara Prats (UPC), Daniel López-Codina (UPC), Anna Vilella (Hospital Clínic-ISGlobal) and Antoni Trilla (Hospital Clínic-ISGlobal), the document provides keys for a better control of the outbreaks and how to estimate the likelihood of an outbreak using data analysis. It also provides tools for early detection and risk assessment.
The phased relaxation of confinement measures has led to an increase in mobility, greater contact between people and the arrival of small numbers of tourists from countries that are currently in an epidemiological situation similar to that of Spain. These developments will lead to an increased risk of infection, especially if people do not carefully follow the recommended personal preventive measures (physical distancing, hand hygiene and mask wearing) and avoid prolonged close contact and crowds, particularly in closed and poorly ventilated spaces. People are being encouraged to avoid the “3 Cs”: closed spaces, crowded spaces and close-contact settings. SARS-CoV-2 is still with us: new cases are reported every day, in smaller but still significant numbers. If the virus is present and we are in contact with each other, the infection will spread.
Can Epidemiological Models Predict Outbreaks?
The answer is no. This pandemic has amply demonstrated the usefulness of epidemiological modelling, including predictive models and models designed to help us understand the epidemiological dynamics of COVID-19 and the different scenarios that could arise. However, it is essential to understand both the scope and the limitations of epidemiological models, which vary a great deal from one phase to the next:
- During widespread community transmission, models can be highly predictive. They are a key decision-making tool because they allow us to compare different scenarios. In health care resource planning, they can be used to quantify the need for special resources such as intensive care beds.
- During containment phases, models lose their predictive power. When effective contact tracing strategies are in place, small outbreaks appear and are contained at the local level. Outbreaks are difficult to predict because their appearance depends essentially on the behaviour of individuals or groups of individuals.
Can We Estimate the Likelihood of an Outbreak Using Data Analysis?
The answer is yes. The primary indicator for assessing the likelihood of an outbreak is the number of active cases, also known as the infectious population. This population cannot be measured with absolute precision, but it can be estimated by counting all cases diagnosed in the last 14 days. The indicator recommended by the European Centre for Disease Prevention and Control is the number of new cases reported per 100,000 population over the past 14 days, also known as 14-day incidence. If the number of active cases is very low, the likelihood of an outbreak is also very low; if the number of active cases is high, the likelihood of a new outbreak is also high.
Other indicators of virus circulation may also be useful for assessing the likelihood of new outbreaks:
- Wastewater analysis. The presence and concentration of viral genetic material in wastewater can be analysed to determine whether SARS-CoV-2 is circulating in a given population.
- Systematic diagnostic testing. Systematic PCR diagnostic testing of a representative or random sample of a population or a specific population group (e.g. health professionals or pregnant women) is a useful screening method for estimating the overall prevalence of the virus because it identifies asymptomatic or presymptomatic cases in addition to symptomatic cases.
Can We Assess the Risk of an Outbreak Getting Out of Control?
The answer is yes. To assess the risk of an outbreak getting out of control, the first thing we need is a good warning system capable of detecting the early signs of an expanding outbreak. The second priority is to identify the factors that may fuel the expansion of the outbreak or help to get it under control. In a given population, the likelihood of an outbreak getting out of control is ultimately determined by the capacity for diagnostic testing and contact tracing. Once the incidence rate rises to a higher levels, the primary care and public health system may no longer be able to follow up infected individuals on a case-by-case basis and keep track of chains of transmission
Key aspects to consider when assessing the risk of an outbreak are:
- Significant increase in case count. It's the first warning sign.
- Consolidation of the rising case count over several days.
- Simultaneous increase in case counts in nearby areas. Can indicate a certain level of community spread.
- Mobility in the affected area. A high level of mobility implies a higher level of interaction among people and can contribute to the emergence of new hotspots in other areas.
- Population density in the affected area. High population density increases the risk of community spread and makes it difficult to trace every chain of contacts
Tools for Early Detection and Risk Assessment
In the current phase of the epidemic, it is essential to work at the smallest possible scale. If containment measures are necessary, it is always preferable to apply them to a single building than to an entire neighbourhood, town, county or province. However, it is difficult to analyse the whole situation thoroughly when working with small numbers. It is important to strike a good balance between scale and analytical capacity.
When detecting outbreaks and assessing the risk that they will spiral out of control, there are three essential indicators to consider:
- Is the number of cases increasing? At what rate? To answer these questions, we must consider several variables.
- How big is the population in which the disease is spreading at this rate? An increase from three to six cases is not the same as an increase from 300 to 600 cases, even though the number of infected people has doubled in both scenarios.
- What is the testing and contact tracing capacity of the system? As long as the number of new cases is within the capacity of the detection and surveillance system, the outbreak can be contained. A good indicator of this capacity is the number of diagnostic tests that can be performed each day per 100,000 population. This number can tell us whether we are dealing with a low-, intermediate- or high-risk area.
These three indicators can be represented in a risk diagram—a visual tool that can help us understand an outbreak.
The Keys to Controlling an Outbreak
- Detect the outbreak early. While the number of cases is still very small, monitor the risk diagrams, identify all (or most) of the people who may be affected and administer diagnostic tests to determine their actual status promptly.
- Identify and trace all contacts. Remember that up to 40% of cases may be asymptomatic. Cases must be isolated and contacts must be quarantined for 10 to 14 days to prevent further transmission.
- Adopt proportionate control measures that are as limited as possible. If necessary, these measures can be scaled up as the outbreak progresses.