Scientists have discovered our immune systems might be inadvertently helping COVID-19 get a foothold in the body, perhaps explaining why it's so easily contracted.
Since emerging in China late last year, the SARS-CoV-2 virus has infected millions of people around the world. The disease it causes - COVID-19 - has killed nearly 200,000 people to date, a toll growing about 4 percent every day.
As a new virus there is no vaccine or proven cure, and no one is naturally immune. Scientists have been scrambling to figure out how it works. It's a close relation to the SARS virus which killed hundreds of people in the early 2000s, and related to other known coronaviruses, but knowledge of its effects on the body and why it's so good at spreading remain a work in progress.
Two separate studies published in the past 24 hours however have shed light on the latter.
The first, conducted by researchers at MIT in the US and published in journal Cell, explains how the body's immune system might be partly to blame.
SARS-CoV-2 infects a victim by attaching itself to an enzyme on the outer layer of cells called ACE2. But the gene which controls ACE2 is activated by a protein called interferon, which the body produces as part of its immune response to infection.
Essentially, the immune system encourages cells to produce an enzyme which SARS-CoV-2 can then use to infect cells.
"The finding suggests that coronaviruses may have evolved to take advantage of host cells' natural defenses, hijacking some proteins for their own use," the team said in a statement.
They also found the cells most likely to be attacked by the virus are found in large numbers in the lungs, nasal passage and the intestine.
This finding is backed up by research led by the Wellcome Trust Sanger Institute in the UK. They used data from the Human Cell Atlas to find where cells containing ACE2 and another enzyme known to be targeted by the virus, TMPRSS2, lived in the body.
They found the largest concentrations lived on the inner lining of the nose. They were also found in the tear duct and the intestine.
"Mucus-producing goblet cells and ciliated cells in the nose had the highest levels of both these COVID-19 virus proteins, of all cells in the airways. This makes these cells the most likely initial infection route for the virus," said Waradon Sungnak, who co-authored the research paper, published in journal Nature Medicine.
Ciliated cells are covered in tiny hair-like projections.
"This is the first time these particular cells in the nose have been associated with COVID-19," said Martijn Nawijn of the University Medical Center Groningen in the Netherlands, who consulted on the paper.
"While there are many factors that contribute to virus transmissibility, our findings are consistent with the rapid infection rates of the virus seen so far. The location of these cells on the surface of the inside of the nose make them highly accessible to the virus, and also may assist with transmission to other people."
The researchers hope the findings will help speed up the hunts for a vaccine and effective treatments.
"There's been an incredible outpouring of information from the scientific community with a number of different parties interested in contributing to the battle against Covid in any way possible," said Alex Shalek, part of the MIT team.
"It's been incredible to see a large number of labs from around the world come together to try and collaboratively tackle this."