Study finds COVID symptoms start two days after exposure

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Study finds COVID symptoms start two days after exposure

The world’s first “human challenge” trial in which volunteers were deliberately infected with COVID-19 has found that people typically develop symptoms very quickly — on average, within two days of being exposed to the virus, according to a new study.

Researchers at Imperial College London who infected 36 people found that symptoms start to develop within about 42 hours, which is earlier than the widely held view that the bug has an incubation period of around five days, Reuters reported.

The virus peaks about five days into infection and at that stage, it is significantly more abundant in the nose than the throat, where the infection first appears, according to the study.

The 36 participants — healthy men and women age 18 to 29 with no immunity to the virus — were monitored at the Royal Free hospital in London, the Guardian reported.

Graph from COVID study
The participants were monitored at the Royal Free hospital in London.
Springer Nature
Sign for masking
The virus peaks about five days into infection.
Christopher Sadowski

The study was carried out using the original SARS-CoV-2 strain before the emergence of the Alpha, Delta and Omicron variants. Its results, which have not yet been peer-reviewed, were published on Springer Nature’s pre-print server.

No serious adverse events occurred, and the human challenge study model was reportedly shown to be safe and well-tolerated among healthy young adults.

Eighteen volunteers became infected, 16 of whom developed mild to moderate cold-like symptoms, including a stuffy or runny nose, sneezing and a sore throat. Some experienced headaches, muscle and joint aches, fatigue and fever.  

Graph from COVID study
Volunteers for the trial were deliberately infected with COVID-19.
Springer Nature
A general view of a "masks are required on public transit" sign
Thirteen participants temporarily lost their sense of smell.
Christopher Sadowski

The 18 all had similar viral loads regardless of whether they developed symptoms, underlining the role of asymptomatic transmission.

 “A lot of people could be walking around shedding virus and not realizing. It’s really marked with this virus,” said Professor Wendy Barclay, head of the department of infectious disease at Imperial College London, the Guardian reported.

Thirteen temporarily lost their sense of smell, but it returned within 90 days in all but three participants.

Graph from COVID study
Researchers at Imperial College London infected 36 people.
Springer Nature

“People in this age group are believed to be major drivers of the pandemic and these studies, which are representative of mild infection, allow detailed investigation of the factors responsible for infection and pandemic spread,” said Professor Christopher Chiu, the trial’s chief investigator.

“Our study reveals some very interesting clinical insights, particularly around the short incubation period of the virus, extremely high viral shedding from the nose, as well as the utility of lateral flow tests, with potential implications for public health,” he said.

“We found that overall, lateral flow tests correlate very well with the presence of infectious virus,” Chiu continued, the Guardian reported.

Graph from COVID study
Symptoms start to develop within about 42 hours.
Springer Nature
Droplet falls from a syringe
The human challenge study model was reportedly shown to be safe.
AP Photo/David Goldman, File

“Even though in the first day or two they may be less sensitive, if you use them correctly and repeatedly, and act on them if they read positive, this will have a major impact on interrupting viral spread,” he added.

The researchers said they plan to start a similar study using the Delta variant, and will share their framework globally to allow other studies that could provide a crucial route to testing new vaccines, antivirals and diagnostics against COVID-19, according to Reuters.

“Scientifically, these studies offer real advantage because the timing of exposure to the virus is always known exactly, therefore things like the interval between exposure and the profile of virus shedding can be accurately described,” said Jonathan Van-Tam, deputy chief medical officer for England, according to the Guardian.

Graph from COVID study
The researchers said they plan to start a similar study using the Delta variant.
Springer Nature

“This important study has provided further key data on COVID-19 and how it spreads, which is invaluable in learning more about this novel virus, so we can fine-tune our response,” he added.

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