Recent case and fatality figures from the European Centre for Disease Prevention and Control (ECDC) show that while recorded Covid-19 cases are spiking in the United Kingdom, France, Spain, Germany and other European countries, deaths are not rising at the same rate.

"Our current estimate is that the infection fatality rate is going up a little bit, but it hasn't come up to anywhere near where we were and that's unlikely to change dramatically unless we see a really surprising increase in the numbers of deaths."

Oke has been tracking Covid-19 fatality rates along with his colleague Carl Heneghan of the Centre for Evidence-Based Medicine and health economist Daniel Howdon.

A wider analysis of data from the US Centers for Disease Control and Prevention by the NYU team shows that across the United States, "6.7% of cases resulted in death in April, compared with 1.9% in September."

Younger, healthier people are getting infected

The most obvious reason for the lower death toll is age.

That means that more than one in 25 care home residents who died since the beginning of the pandemic did so because of Covid-19.

With more young people getting infected, the overall fatality rate has dropped, but this doesn't mean the virus itself has become any less deadly.

"The variation in severity of Covid-19 illness is really due to individual host immune responses together with age, sex, ethnicity and certain pre-existing medical conditions," he added.

Treatments are getting better

The demographic shift may have contributed to the lower death toll, but experts suspect the fact that healthcare providers are now more experienced in dealing with Covid-19 patients is another factor.

"If you only test the symptomatic cases you may massively underestimate the number of infecteds if the proportion of asymptomatic infecteds is large," Tang said, adding that some studies suggest that up to 60% to 70% of Covid-19 cases may be asymptomatic.

"So the reported [case fatality rate] may be disproportionately high at the start of the pandemic but then drops later as the pandemic progresses, as we test more asymptomatic cases to 'dilute' down this apparent death rate," Tang added.

The main danger, Tang said, is that more widespread testing of the less vulnerable younger population may mask mortality rates in those who are older or have underlying conditions.

"The time difference between when we think people are getting infected and when they might die, on average it's about three weeks, but what we're seeing is that the [infection fatality rate] is staying lower even as the preceding infections have gone up," said Oke.

This story has been updated to include US CDC figures.