Interrupted-time series analyses are not useful for the analysis of the Covid-19 pandemic

The analysis of the Covid-19 pandemic has produced a large amount of scientific output. One approach that has been used to analyze trends of Covid-19 has been interrupted time-series analysis. The reason that this method does not work on novel outbreaks is fairly simple for infectious disease epidemiologists, this method assumes linearity in the trends of a diseases, meaning, the number of cases will increase linearly from the date before.

As a showcase, the following figures show the non-linear trends of symptomatic Covid-19 cases in Colombia, for each capital city:

These are simply outbreaks dynamics working here. Interrupted time-series analyses can be useful for endemic infectious diseases, and is a method commonly used for assessing the impact of interventions in an ecological level. Novel outbreaks of infectious diseases cannot be analyzed using interrupted time-series analysis because the dynamics of the disease depend on the number of susceptibles, contact rates, and the transmissibility of the pathogen.

The preprint by Moreno-Montoya et al., published in medRxiv recently, uses interrupted time-series analyses to evaluate the effect of social protests on infectious disease dynamics in Colombia. The study is a good effort but suffers from the error of not taking into consideration the number of susceptibles, seroprevalence, and contact rates in the counterfactual. The plots above show several cities in Colombia without much protests, having a reduction of cases in the last month. The red line is 5 days after the start of protests in the country. Cartagena, for example, did not have sustained and large protests during the timeframe, and the trends show a decline in symptomatic cases.

Despite this study is not fit to address the question, we need to provide context into the hypothesis that social protests increased the number of symptomatic cases. The protests started in late-April, where there were around 70 thousand deaths already, the number of cases were climbing in most cities, and the government is removing the restrictions in place to handle the pandemic. The lack of evidence can not be an excuse for inaction. The death toll is already almost half of deaths for all causes in the country in most recent years, a tragedy that could had been reduced with better measures.

The political discussion of the blame-game is not something I would write here. But high-quality evidence is necessary to improve as a society, and the study from Moreno-Montoya et al. is not adequate to answer this question.

Going forward, the government needs to address the situation head on. There is a healthcare crisis caused by the Covid-19 pandemic in Colombia no one is willing to acknowledge. Vaccines have suffered from a slow rollout. Cases are still rising despite the protests are largely gone, there is shortage of oxygen and other medications top handle the pandemic, there is mental exhaustion in the healthcare personnel, and most importantly, people are still dying in very large numbers from Covid-19.

The reopening of the economy is necessary, but the government can and should be more mindful of the effects of some measures on the death toll and circulation of the SARS-CoV-2 virus.