Effective surveillance of key non-symptomatic hosts to prevent local outbreaks during outbreaks

A team of researchers in the UK and Israel have warned that during outbreaks of infectious diseases such as coronavirus 2019 (COVID-19), effective surveillance of non-symptomatic hosts is essential to prevent local outbreaks.

Using COVID-19 as a case study, the team used a ‘branching process model’ to estimate the risk of imported cases leading to local outbreaks in a host population.

The study showed that trying to only increase surveillance of newly arrived symptomatic individuals is not an optimal approach to reducing this risk of a local outbreak.

A strategy that combines increased surveillance of symptomatic cases with efforts to identify and isolate non-symptomatic cases is the most effective approach to reducing this risk, say the researchers.

This combined strategy is the most effective, even when the effort required to identify non-symptomatic cases is much greater than that required to identify symptomatic cases, they add.

The team – from Oxford University, Tel Aviv University, London School of Hygiene & Tropical Medicine and Imperial College London – say this underscores the importance of allocating resources to find non-symptomatic guests as well as cases. symptomatic during outbreaks of infectious infections. pathology.

“During outbreaks of COVID-19 and other infectious diseases, effective surveillance for non-symptomatic hosts can be crucial to prevent local outbreaks,” say Francesca Anne Lovell-Read and colleagues.

A pre-printed version of the research paper is available on medRxiv* server, while the article is peer reviewed.

Varying the base reproduction number 0 from its base value (0 = 3). The graphs are analogous to Figure 3D, showing strategies for minimizing the surveillance effort required to achieve a pre-specified level of risk (an “acceptable” local outbreak probability). The red dashed lines represent the contours along which the probability of a local outbreak is constant, as indicated; the red circles indicate the points along these contours where the total 7 + 8 surveillance effort is minimized. The white line indicates the optimal strategy to follow if the pre-specified risk level is reduced. Apart from 0 and (which is changed in each panel to set the value of 0), all parameters are kept fixed at the basic values ​​indicated in Table 1. A. 0 = 1 · 5. B. 0 = 2. C. 0 = 2 · 5. D. 0 = 3 (baseline). E. 0 = 3 · 5. F.

Non-symptomatic hosts present outbreak surveillance challenges

During infectious disease outbreaks, two key questions are whether infected cases traveling to new locations will cause local outbreaks and whether surveillance and control interventions in these new locations could reduce this risk of outbreak.

During any outbreak, one factor that can hinder surveillance and control interventions is the possibility that presymptomatic or asymptomatic (here referred to as non-symptomatic) transmit a pathogen.

“Determining the extent to which non-symptomatic individuals contribute to transmission is essential at the onset of emerging outbreaks of a new pathogen,” the team writes.

Using COVID-19 as a case study to investigate

In the case of COVID-19, the estimated average incubation period is approximately 5-6 days, during which non-symptomatic transmission can occur.

Using COVID-19 as a case study, Lovell-Read and colleagues used a branching process model to assess the risk of local outbreaks occurring through non-symptomatic transmission among imported cases.

Branching process models are used to assess whether newly arrived cases among a host population will cause a local outbreak and to predict whether potential control measures would be effective.

Assuming that imported cases are indeed isolated, the team investigated the effectiveness of surveillance targeting non-symptomatic individuals, symptomatic individuals, or a combination of both groups, when surveillance resources are limited.

What did the study find?

The modeling study showed that increased surveillance of non-symptomatic individuals could be an important contributor to the risk of local outbreaks during outbreaks.

“Trying to increase surveillance of symptomatic hosts alone is generally not the optimal strategy to reduce the risk of an outbreak,” the team says.

Optimal reduction in local outbreak risk was almost always achieved when a combined strategy was adopted that involved increased surveillance of symptomatic and non-symptomatic hosts.

“This remains the case even though the surveillance effort required to find non-symptomatic infected individuals is significantly greater than the effort required to find symptomatic individuals,” write Lovell-Read and colleagues.

What are the implications of the study?

“Adopting a strategy that combines improved surveillance of symptomatic cases with efforts to find and isolate non-symptomatic hosts leads to the greatest reduction in the likelihood of imported cases to initiate a local epidemic,” the team writes.

The researchers add that the findings should apply to outbreaks of any pathogen for which non-symptomatic individuals significantly contribute to transmission.

Effective surveillance of non-symptomatic individuals is essential to prevent local outbreaks during COVID-19 outbreaks and other infectious disease outbreaks, they say.

“Dedicating surveillance resources to search for non-symptomatic cases can be an important component of public health measures that aim to prevent local outbreaks,” the team concludes.

*Important Notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be considered conclusive, guide clinical practice / health-related behavior, or treated as consolidated information.