Scientists Weigh In on Virus Load, Dose, and Shedding

There are lots of opinions on the spread of COVID-19 and the virus load, dose, and shedding.  Recently some of the leading authorities on infectious diseases has weighed in on these issues.

Prof Wendy Barclay, Action Medical Research Chair Virology, and Head of Department of Infectious Disease, Imperial College London, said:

“In general with respiratory viruses, the outcome of infection – whether you get severely ill or only get a mild cold – can sometimes be determined by how much virus actually got into your body and started the infection off.  It’s all about the size of the armies on each side of the battle, a very large virus army is difficult for our immune systems army to fight off.

“So standing further away from someone when they breathe or cough out virus likely means fewer virus particles reach you and then you get infected with a lower dose and get less ill.  Doctors who have to get very close to patients to take samples from them or to intubate them are at higher risk so need to wear masks.

“The fewer people in the room, the less likely it is than one person is coughing or breathing out infectious virus at any one time, so mixing with as few people as possible is the safest way.

“But there is no evidence for any suggestion that if everyone in a family is already sick they can they reinfect each other with more and more virus.  In fact for other viruses once you are infected it’s quite hard to get infected with the same virus on top.”

Professor Willem van Schaik, Professor in Microbiology and Infection at the University of Birmingham, said:

“The minimal infective dose is defined as the lowest number of viral particles that cause an infection in 50% of individuals (or ‘the average person’). For many bacterial and viral pathogens we have a general idea of the minimal infective dose but because SARS-CoV-2 is a new pathogen we lack data. For SARS, the infective dose in mouse models was only a few hundred viral particles. It thus seems likely that we need to breathe in something like a few hundred or thousands of SARS-CoV-2 particles to develop symptoms. This would be a relatively low infective dose and could explain why the virus is spreading relatively efficiently.

“On the basis of previous work on SARS and MERS coronaviruses, we know that exposure to higher doses are associated with a worse outcome and this may be likely in the case of Covid-19 as well.  This means that health care workers that care for Covid-19 patients are at a particularly high risk as they are more likely to be exposed to a higher number of viral particles, particularly when there is a lack of personal protective equipment (PPE) as is reported in some UK hospitals (https://www.theguardian.com/society/2020/mar/22/nhs-staff-cannon-fodder-lack-of-coronavirus-protection).

“It seems unlikely that people can pick up small numbers of viruses from others (e.g. in a crowd) and that will tip the infection over the edge to become symptomatic as that must happen around the same time. In the current lockdown situation this seems even less likely as gatherings of more than two individuals are banned. Because the infectious dose is probably quite low, it is more likely that you will be infected by a single source rather than from multiple sources. Transmission can take place through small droplets in the air (like the ones that are produced after sneezing and which stay in the air for a few seconds). You can breathe in these droplets or they can land on surfaces. Unfortunately, SARS-CoV-2 survives reasonably well on most surfaces, so if somebody touches these and then touches their mouth or nose, there is a very real risk that they will be infected with the viruses. This is the main reason why hand washing is promoted as a precautionary measure.”

From Prof Richard Tedder, Visiting Professor in Medical Virology, Imperial College London:

What is “viral load”?

“This is a specific term used in medical virology which usually refers to the amount of measurable virus in a standard volume of material, usually blood or plasma. It is very commonly used to define how HIV responds in a patient to antiviral drugs; a patient taking such drugs would be pleased to know that their ‘viral load’ is reduced.”

What does viral load mean for Sars CoV 2 (aka Covid19 virus)?

“It is probably better to use the term ‘viral shedding’ which is actually in effect influenced by the amount of virus in the material being shed by an infected patient. In practice one could say that the virus load generated by the patient in whatever excreta they shed defines ‘shedding’ and its risk.

“From looking broadly at the overall data on the material which comes from a nose swab the amount of virus varies over a 1 million fold range. This is probably influenced by the stage of the disease, the efficiency with which the infection has colonised the patient at the time of sampling, and the amount of nasal sample on the swab. The amount of virus which comes from an infected person is influenced by two factors: the ‘load’ in the excreta and the volume of the excreta.

Why does the amount of virus shed matter?

1. “The inoculum, i.e. the infecting dose of virus is more likely to lead to infection in the “recipient” the higher the amount of the virus there is in the excreta.

2. The virus will survive and remain infectious outside the body, as viruses do; BUT infectivity will fall away with time. How quickly this fall occurs is measured as the time taken for virus infectivity to reduce by half. This is termed ‘half life’ or T1/2 and for this virus is measured in hours. In fact this is best thought of as ‘rate of decay’.

3. The rate of decay is fastest on copper with a T1/2 around 1 hour, in air as an aerosol T1/2 is also around 1 hour, cardboard is 3 and 1/2 hours, plastic and steel T1/2 is around 6 hours.

“For example, if one million viruses were placed on various surfaces it would require 20 half lives to become undetectable and non-infectious, so 20 hours if in an aerosol, 20 hours on copper, 60-70 hours on cardboard and finally 120-130 hours on plastic and steel.

“Of course, when one deals with infectivity rather than detectability, extinguishing infectivity is far quicker.  Studies with cultured virus starting at relatively high levels have shown loss of infectivity within around 12-15 hours on copper, under 10 hours on cardboard, around 50 hours on steel and 70 hours on plastic. The data for infectivity in aerosols were not comparable and were of a different time course.”

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