Self-reports of smell and taste changes can be earlier markers of the spread of the novel coronavirus than indicators based on hospital emergency room (ER) consultations, according to an international team of researchers.

The team's study, published in the journal Nature Communications, noted a decline in self-reports of smell and taste changes as early as five days after lockdown enforcement, with faster declines reported in countries that adopted the most stringent lockdown measures.

"In response to the COVID-19 pandemic, many governments have taken drastic measures to prevent their intensive care units from becoming overwhelmed with patients," said study co-author John Hayes, professor of food science at The Pennsylvania State University (Penn State) in the US.

"Our research suggests that an increase in the incidence of sudden smell and taste change in the general population may indicate that COVID-19 is spreading," Hayes said.

The researchers believe the findings could help decision-makers take important measures at the local level, either in catching new outbreaks sooner, or in guiding the relaxation of local lockdowns, given the strong impact of lockdown on economic and social activities.

In the study, the scientists used data from the Global Consortium for Chemosensory Research survey -- a global, crowd-sourced online study deployed in more than 35 languages.

The team also particularly assessed data from the French government -- which beginning on May 7, 2020, has been categorising various geographical areas of the country as red or green, depending on their COVID-19 prevalence.

Compared to green areas, they said the red areas were characterised by higher active circulation of the virus, higher levels of pressure on hospitals and reduced capacity to test new cases.

The researchers then compared data from France with data from Italy and the UK, each of which implemented lockdown measures at different times and with different levels of stringency.

"Our primary aim was to test the association between self-reported smell and taste changes and indicators of pressure in hospitals, such as COVID-related hospitalisations, critical care resuscitation unit admissions and mortality rates, for each French administrative region over the last three months," said Veronica Pereda-Loth, a study co-author from the Universite Paul Sabatier Toulouse III in France.

"Our secondary aim was to examine temporal relationships between the peak of smell and taste changes in the population and the peak of COVID-19 cases and the application of lockdown measures," Pereda-Loth said.

The study found that smell and taste changes were better correlated with the number of COVID-19 admissions to hospitals than France's current governmental indicators, which look at the ratio of ER consultations for suspicion of COVID-19 to general ER consultations.

According to the scientists, the peak onset of changes in smell/taste appeared four days after lockdown measures were implemented.

In contrast, they said the governmental indicator based on ER consultations peaked 11 days after the lockdown.

"Our findings are consistent with emerging data showing that COVID-19-related changes in smell and taste occur in the first few days after infection," Hayes said.

"They suggest that self-reports of smell and taste changes are closely associated with hospital overload and are early markers of the spread of infection of SARS-CoV-2," he added.

The scientists believe potential outbreaks and the short-term efficacy of a lockdown could be monitored by tracking changes in smell and taste in the population.