Coronavirus: New theory on why the COVID-19 death rate isn't rising, despite accelerating spread

Scientists looked at nasopharyngeal swab data from hospitalised patients in Detroit between April and June. Photo credit: Getty

Since January, nearly 1 million people have died in the COVID-19 pandemic. But though the rate of infection is increasing, the rate of reported deaths isn't.

A typical day in September has seen 300,000 new confirmed infections, more than triple recorded in April - but 8000 people a day were dying then, compared to about 5500 now. 

Various explanations have been given, including more younger people catching the virus now who are less likely to die, and more robust testing regimes picking up more cases.

The former has been backed by epidemiologists here and the World Health Organization, which in August urged youth to give up their partying ways until the pandemic was over. And the latter theory is supported by blood testing which has generally found exposure to the virus in far more people than have reported symptoms.  

And doctors over the past eight months have gotten better at treating the disease, figuring out what works and what doesn't, even as a foolproof cure remains elusive.

But researchers in the US have come up with a new explanation for the declining case-to-fatality ratio (CFR) - people aren't getting as badly infected as they were early in the pandemic. 

Scientists looked at nasopharyngeal swab data from hospitalised patients in Detroit between April and June.

They found between April 4 and June 5 there was a "progressive decline" in the average viral load which matched a decline in the CFR. 

While almost half the patients who had a 'high' viral load died, only 32 percent of those with an intermediate load died, and 14 percent with low. Over the three months, more and more patients had low viral loads. 

"This suggests an association between initial viral load and mortality," the Wayne State University scientists said. 

Said El Zein, who led the research, said the exact reasons for the declining viral load were unclear - but he had a theory.

"Rapid implementation of social distancing measures, lockdown and widespread use of facemasks may have contributed to a decrease in the exposure to the virus."

Meaning as time went on and people started following distancing guidelines and wearing masks, even when they were exposed to the virus, they didn't contract as much. 

The research was presented at a European Society of Clinical Microbiology and Infectious Diseases conference this week, and is yet to undergo peer review.