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Infectious respiratory diseases spread when a healthy person comes in contact with virus particles expelled by someone who is sick — usually through a cough or sneeze. The amount of particles a person is exposed to can affect how likely they are to become infected and, once infected, how severe the symptoms become.

The amount of virus necessary to make a person sick is called the infectious dose. Viruses with low infectious doses are especially contagious in populations without significant immunity.

The minimum infectious dose of SARS-CoV-2, the virus that causes Covid-19, is unknown so far, but researchers suspect it is low. “The virus is spread through very, very casual interpersonal contact,” W. David Hardy, a professor of infectious disease at Johns Hopkins University School of Medicine, told STAT.

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Alex Hogan/STAT

A high infectious dose may lead to a higher viral load, which can impact the severity of Covid-19 symptoms.

Viral load is a measure of virus particles. It is the amount of virus present once a person has been infected and the virus has had time to replicate in their cells. With most viruses, higher viral loads are associated with worse outcomes.

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“The more viral particles that get into the lungs, the more damage to the lungs that is probably happening,” said Hardy.

One study of Covid-19 patients in China found that those with more severe symptoms tended to have higher viral loads.

“It’s not proven, but it would make sense that higher inoculating doses will lead to higher viral loads, and higher viral loads would translate into more pathogenic clinical courses,” said Dan Barouch, director of the Center for Virology and Vaccine Research at Beth Israel Deaconess Medical Center.

Alex Hogan/STAT

People with higher viral loads may also shed more whole viruses, which makes them more contagious, compounding the danger of spreading disease more widely.

If exposure to higher doses, or even frequent low doses, of SARS-CoV-2 does lead to worse health outcomes, there are significant implications for health care workers who are routinely exposed to Covid-19 patients.

“Someone caring for large numbers of patients on the wards, if they’re not wearing PPE [personal protective equipment], there might be a high frequency of exposure as well as a high dose of exposure,” Barouch said.

In Italy, a country particularly hard-hit by the virus, about 9% of reported cases were health care workers. Here in the U.S., 10% of Covid-19 cases in California were health care workers, according to the California Department of Public Health.

  • Dear Alex – your article makes eminent sense – the high rate of severe infection, even in younger healthcare workers, suggests that the amount of initial exposure may have prognostic implications. The obvious corollary is whether exposure to very low viral load, initially, leads to less severe forms of the infections and my be an approach to lowering severity of morbidity or mortality.

    • Ivan – I agree with your Comment, with the caveat we do not know for certain the numbers of youth, same age as the health care workers, who get very sick with this. But the numbers we have been given suggest they are much, much lower. Maybe the health care people are pushing themselves very hard, and that is making them sicker also.
      BUT, the theory of initial infection dose being related to how sick one eventually becomes would seem to make sense in a kind of – I am searching for words – a kind of straight line virus -where it reproduces steadily, and the typical immune response is kind of straight line as well – well, does not have to be straight line- a graph with the curve that keeps going up until the immune system kicks in it starts going down.

      The problem I have with thinking this virus behaves that way is again and again I hear reports of people getting sick, being identified as sick, “resting comfortably at home” as was said of Boris Johnson, but there are many like that – and then suddenly getting much worse.

      If the virus kept increasing in the body, then peaked and decreased, while illness is proportional to viral load, it seems to me you would not see this pattern. Please let me know your thoughts.

  • Why don’t we see a comparison charts comparing common flu to Covid 19. We need to see a historical baseline.

    • Rory -I am not a doctor or virologist, but from following the news about this virus intensely starting about 4 weeks back, and reading up on virology, I wouid say very emphatically we do NOT need to see an historical baseline in regards to how the disease progresses in patients. Or for that matter, in how it is progressing as an epidemic compared to influenza.
      As best I can tell, it’s more the opposite, no one should be making assumptions of what this NOVEL virus will do based on what completely NON NOVEL viruses do.

    • Rory -I am not a doctor or virologist, but from following the news about this virus intensely starting about 4 weeks back, and reading up on virology, I wouid say very emphatically we do NOT need to see an historical baseline in regards to how the disease progresses in patients. Or for that matter, in how it is progressing as an epidemic compared to influenza.
      As best I can tell, it’s more the opposite, no one should be making assumptions of what this NOVEL virus will do based on what completely NON NOVEL viruses do.

  • Why can’t all the big name pharmaceutical companies join heads and come up with an effective drug treatment. So far I am see precautions but not solutions !

  • While I appreciate the effort, this article offers no useful information. It is all speculation.

  • What should be the advice of people who are infected with the virus in places where are no hospital? Because people are recovering . What are they doing to recover?

  • Workers in a confined setting such as transit authority and grocery store clerks are dying from exposure to this virus at an alarming rate. So is the amount of virus particles that are getting into these peoples lungs playing a factor as why the deaths in these occupations are rising?

  • Can you contract by clothing? O where are my manners.. Thank you for informant. How wonderful to to meet.

  • So why not purposefully dose healthy people with no known risk factors a VERY small dose of the virus, perhaps less than 5 cells, and let their immune system fight it off, so that they either have no symptoms, or only a mild case, but resulting in immunity to a more serious dose of the virus?

    • Because the amount of viral exposure necessary to trigger an immune response would also be enough to actually infect you

    • Yes, that is what I am suggesting; purposefully infect healthy people with a very low dose of the virus. Was my suggestion not clear? By infecting those people, they would experience the effects of exposure; what is not to understand? But since the quantity of viral particles is a critical aspect of what infection response a person experiences, a very low dose would seem to produce the lowest possible infection response.

    • Infectious dose, and viral load are not the same thing. It’s not known at this time what the minimum infectious dose is, or whether that transposes to a viral load of the same magnitude once infected. Your suggestion is incorrectly making a presumption that a low infectious dose maps to a low viral load which is not proven.

    • Mike D = I am interested in your thoughts – the theory of greater viral load leading to greater disease seems reasonable, and I have heard the claim that the severe respiratory distress does related to how much virus is in the lungs, and that in turn relates to how bad the damage due to excessive inflammation is.
      But, in many patients, the disease progression, from what we read is, someone gets noticeably sick – they appear to be recovering – then they go downhill fast = and, they may still survive, but at the least they are clearly in danger. Boris Johnson was just one of many I have read with this pattern (of course he may have lied about the progress of his illness, but it seems to be a common pattern)

      So, if you can be only slightly sick for a week or more then get much worse, I would think the amount of virus in you after a full week is not very closely correlated with how much you originally took in. Or, if it was, then illness is not closely correlated with viral load, because if that was true, then your would get steadily worse after infection.
      I think I read a report most of the very sick have an excessive inflammation in their lungs and that was correlated with virus load in the lungs.
      I also read that old folks, maybe it was the sickest old folks -had the most virus. I am thinking the extreme illness is very highly correlated, maybe actually dependent, on cytokine storm, and the cytokine storm takes some time to brew, not because the virus is replicating, but because the immune response, and producing enough antibodies, takes time to kick in.
      The claim the people who do not know they are sick have the fewest antibodies. I would say, perhaps the fewest IDENTIFIED antibodies? Maybe the known antibodies are what does the harm in most cases?

  • So if a person sneezes on the other part of your body and it dries up does it mean the virus also went into the body ?

  • Okay. So for smallpox when they did her immunity kids developed tolerance and not immunity.

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