Researchers in the UK and Australia have shown that epidemiologic models accounting for people’s behavior outside of measures introduced to control the coronavirus disease-19 (COVID-19) pandemic improve estimates of how effective such interventions have been.
The study was conducted followed the team’s discovery that physical distancing increased in the UK before non-pharmaceutical interventions (NPIs) were implemented and soon decreased once they had been implemented.
The researchers warn that if independent behavioral choices are not considered, the degree of physical distancing that occurs without NPIs could be underestimated and the effectiveness of these measures overestimated.
The team – from the University of Oxford, the University of London, and the University of Technology Sydney – estimated the effect that NPIs have on the epidemic curve after considering individuals’ choices to engage in physical distancing independently of NPIs.
After re-designing the Susceptible, Exposed, Infectious, Removed (SEIR) model to account for behavioral choices (BeSEIR model), Georgios Baskozos and colleagues found that predictions regarding the number of infection cases without NPIs were significantly lower than those generated by standard SEIR.
The BeSEIR predictions also showed that even without NPIs, the proportion of cumulative infections would still not be enough for the epidemic to resolve through population or herd immunity.
This news article was a review of a preliminary scientific report that had not undergone peer-review at the time of publication. Since its initial publication, the scientific report has now been peer reviewed and accepted for publication in a Scientific Journal. Links to the preliminary and peer-reviewed reports are available in the Sources section at the bottom of this article. View Sources
A pre-print version of the paper is available on the medRxiv* server, while the article undergoes peer review.
Many countries have relied on NPIs to help control COVID-19 pandemic
In the absence of any vaccine to protect against COVID-19 during most of 2020, many governments introduced NPIs in efforts to bring the pandemic under control.
Enforceable NPIs range from the closure of public spaces and shops to banning social interactions outside of households and forbidding unnecessary travel.
These interventions aim to reduce disease spread by lowering the reproductive number (number of secondary infections caused by one infected person) through limiting contact between individuals.
Epidemiological models aiming to assess the effectiveness of NPIs are designed to establish how the various policies reduce this reproductive number.
In standard models, this parameter is assumed to be constant at first, before then changing in response to NPIs.
Data have shown people reducing contact prior to NPIs being introduced
“However, data which capture mobility levels of individuals show that in a number of countries, including the UK, people reduced the number of visits and duration of stays (which are related to physical distancing practices) before the NPIs are made and in excess of these measures,” say the researchers.
These observations help to explain why some epidemiological models that account for behavioral changes over and above NPIs show that the effective reproduction number becomes at least partly endogenous.
“However, these works are theoretical and have not been applied to data sets related to COVID-19 so far,” writes the team.
The researchers argue that to be able to assess the interventions accurately, it is essential to account for behavioral changes regarding physical distancing that are the result of both NPIs and individuals’ choices outside of NPIs.
What did the researchers do?
The team estimated the effect that reports of daily confirmed cases had on people’s behavior and the effect that NPIs had on physical distancing, after accounting for this behavioral component. This information was then used to create a BeSEIR model that was applied to different simulated scenarios regarding NPIs and the potential effects of lifting the measures.
The effects of NPIs were considered across three distinct periods: before advice about reducing contact was given, between advice being given and lockdown, and after lockdown.
The results were then compared with those generated using a standard SEIR model.
What did the study find?
When the researchers used the BeSEIR to test whether reports of confirmed daily cases could have contributed to behavioral changes, they identified high correlations across all three periods.
People have been making physical distancing choices using the available information regarding the number of cases,” says the team. “This incorporates a feedback effect between confirmed cases and average number of contacts between individuals, which means that the reproduction rate is (partly) endogenous.”
The BeSEIR predictions regarding the number of infections were several orders of magnitude lower than those generated using the SEIR model.
Furthermore, the standard SEIR model significantly overestimated the effectiveness of NPIs.
If individual behavior is not taken into account, the levels of physical distancing without measures can be underestimated and similarly the effectiveness of measures can be overestimated,” says Baskozos and colleagues.
The BeSEIR predictions also showed that even if no NPIs are introduced, the percentage of the cumulative infections within one year would not be sufficient for the epidemic to resolve through herd immunity.
“Without taking into account the behavioral component, the epidemic is predicted to be resolved much sooner than when taking it into account,” warns the team.
This news article was a review of a preliminary scientific report that had not undergone peer-review at the time of publication. Since its initial publication, the scientific report has now been peer reviewed and accepted for publication in a Scientific Journal. Links to the preliminary and peer-reviewed reports are available in the Sources section at the bottom of this article. View Sources
Article Revisions
- Apr 3 2023 - The preprint preliminary research paper that this article was based upon was accepted for publication in a peer-reviewed Scientific Journal. This article was edited accordingly to include a link to the final peer-reviewed paper, now shown in the sources section.
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