Current state of COVID knowledge

Current state of knowledge, 29th November

US hospitalisations

Infection

Human infectious dose, SARS-CoV-2, unknown by all exposure routes

However

Based on human exposure to other coronaviruses

and animals exposure

the dose at which 50% of humans become infected is 10 to 1,000 plaque-forming units (PFU)

UK, human exposure trials in January 2021 to identify the infectious dose of SARS-CoV-2

Golden Syrian hamsters

Exposure to 56,000 PFU via the intranasal route developed clinical symptoms

Immunosuppressed, severe clinical symptoms (including death) after exposure to 100-10,000 PFU

We need to know

Human infectious dose;

by aerosol

surface contact (fomite)

fecal-oral routes

other potential routes of exposure

Most appropriate animal model(s) to estimate the human infectious dose for SARS-CoV-2

Does exposure dose determine disease severity?

Transmission

Exhaled breath may emit 105-107 genome copies per person per hour

100, 000 to 10,000,000

Droplets

Aerosols

Fomites

Close = a combined total of 15 minutes within 6 feet of an infected person in a 24-hour period

Infectious for 1-3 days prior to symptom onset, 40%

Asymptomatics, 12%

Five days after symptom onset

Asymptomatic individuals can transmit disease as soon as 2 days after infection

Asymptomatic children may have substantially lower levels of virus in their upper respiratory tracts than symptomatic children

In the US, symptomatic index cases resulted in transmission to approximately 53% of household members

SARS-CoV-2 may be spread by conversation and exhalation

Most transmission indoors, offices, restaurants, bars, gyms

Clusters are often associated with large indoor gatherings

Undetected cases play a major role in transmission

Most cases are not reported

Individuals who have clinically recovered but test positive for COVID-19 are unlikely to be infectious

We need to know

Relative contribution of different routes of transmission (e.g., fomites, aerosols, droplets)

How common is transmission from bodily fluids like semen, urine, and feces?

How infectious are young children compared to adults?

What is the emission rate of infectious SARS-CoV-2 particles while breathing, talking, coughing, singing, or exercising

taking into account variation in viral load in the upper and lower respiratory tract?