The day a person starts to show symptoms of COVID-19 is also the day they're most contagious, a new study has found.
But if you catch the virus off someone who isn't showing symptoms, there's some good news too - there's a good chance you won't have any either, reiterating the value of getting vaccinated.
"Much remains unknown about the transmission dynamics of COVID-19," the study, published in journal JAMA Internal Medicine, says. "How the severity of the index case and timing of exposure is associated with disease in close contacts of index patients with COVID-19 and clinical presentation in those developing disease is not well elucidated."
Researchers in China and the US looked at 8852 close contacts of 730 known cases, and found the most infectious period began about two days before the onset of symptoms, to three days after.
The longer a person spent around someone, the more likely they were to pick it up - even more so if the exposure was in that six-day period, with the risks peaking on the day symptoms appeared.
One of the most confounding aspects of the COVID-19 pandemic is that some people don't get sick at all, but can still pass the virus on.
"We found that contacts exposed to asymptomatic index patients were less likely to develop COVID-19, and, given infection, were more likely to be asymptomatic," the study found.
"This result suggests that there may be a dose-response association between severity of the index patient's case of COVID-19 and clinical presentation among contacts. If confirmed in other studies, this result may suggest additional secondary benefits associated with reducing case severity of individuals with COVID-19 through vaccination or prompt diagnosis and treatment."
In other words, since the vaccine reduces the likelihood of serious illness in the case of breakthrough infections - which the Delta variant appears to be quite capable of - it's likely anyone who catches the virus off a vaccinated person will likely experience a less serious illness, vaccinated or not.
Though this study didn't track mask use, it did find evidence frontline healthcare workers were at lower risk of developing COVID-19, "which may be owing to mask use". Previous research has suggested people who contract the virus while wearing a mask are less likely to develop serious illness presumably because they breathe in fewer viral particles.
The data in this latest study was collected in 2020, before the emergence of the more infectious Delta variant. A study published last week found people infected with Delta become contagious earlier than those with earlier strains, increasing the chance of presymptomatic spread.
