Figure 1. As time goes by, severity declines. Time (days after first case) correlates negatively with the duration of symptoms (in days) for patients younger than 60 years (circles) and older than 59 years (triangles) during the first COVID-19 outbreak in an isolated population.
Case fatality and thus possibly also disease severity of COVID-19 patients may be linked with length of follow up period (Rossi et al., 2020). However, in our small study population follow up is essentially continuous. Possible explanations for the observed decline in disease severity are improved anti-inflammatory treatments towards the end of the outbreak, changes in population susceptibility or pathogen virulence, less severe cases showing up later in time, or an effect of lockdown on viral burden or infective doses with subsequent effects of disease severity.
The COVID-19 pandemic presents numerous challenges to epidemiologists and health authorities. The ongoing de-escalation process in Spain will provide information on the most likely mechanisms explaining our observation. If the decline in severity is due to the lockdown, case severity and duration are expected to grow during de-escalation. In this case, only effective contact tracing followed by testing and case isolation should serve to control further outbreaks. However, if host- or pathogen-related factors are involved, this would represent good news since it might facilitate softer disease control interventions. Nevertheless, these observations will contribute to combating the current coronavirus pandemic, its aftermath and potential future clustered outbreaks or comparable healthcare emergencies.
Contributors
FR and CG planned the study. Data were collected by FR and CG. Data analysis was led by LD and JdF. All authors interpreted the study findings, contributed to the manuscript, and approved the final version for publication.
Declaration of interests
We declare no competing interests.
Acknowledgments
This study had no specific funding. We would like to thank the local veterinarian Javier Camarena and the municipality of Horcajo de los Montes for information provided. We acknowledge UCLM support to Grupo SaBio.
Ethical statement
The authors confirm that the ethical policies of the journal, as noted on the journal’s author guidelines page, have been adhered to. No ethical approval was required as this study was based on official records and anonymized. FR had full access to all the data in the study and all authors had final responsibility for the decision to submit for publication.
Data availability
The data that support the findings of this study are available from the corresponding author upon reasonable request.