COVID-19, Don't Miss

How much of the coronavirus does it take to infect a person?

September 9, 2020

Scientists are now speculating if this phenomenon, which may reduce the “infectious dose” of the virus that people are exposed to – can help explain why hospitalizations and deaths are not tracking confirmed cases as closely as they were in Spring.

If they were right, this has important implications for epidemiological modeling and how we assess our hazard and behaviors. For example, where to wear a mask, when to go back to the office, and even what bed to sleep in are all questions that it could inform.

“The initial dose of virus and the amount of virus an individual has at any one time might worsen the severity of Covid-19 disease,” ventured members of the Oxford Covid-19 Evidence Service Team in March. “The amount of virus exposure at the start of infection – the infectious dose – may increase the severity of the illness and is also linked to a higher viral load [in infected patients].”

Concrete evidence is now being built to support this view from the animal, human, and modeling studies, new and old.

Firstly, a clear link has been recognized between the number of virus patients have in their system, the viral load, and the severity of illness. A study from China has shown that a viral load is higher in patients with more severe disease. An American study was published last month has found that “viral load at diagnosis” was an “independent predictor of mortality” in hospital patients’ meaning; the higher the load of the virus, the bigger chance of dying.

Considering this, the evidence is less clear cut, but it is rising. In May, a study was published by scientists at Public Health England’s National Infection Service at Poton, Down, Salisbury, in which they gave ferrets varying doses of Sars-Cov-2 and resulted in a clear difference in the outcomes.

Animal test subjects were given high and medium doses and contracted the virus and underwent many of the same ailments as humans. But then again, a low dose “appeared to result in infection of only one ferret.” It also escaped the disease’s worst effects, with no damaging of its lungs or fatigue reported.

We should not be surprised if the infectious dose of SARs-Cov-2 exposed to will increase the risk of severe disease. Francois Balloux, a geneticist and epidemiologist, pointed out, “There are loads of examples in the literature for symptom severity being dose-dependent for plenty of [other] bugs.”

The most palpable is the flu. A study of influenza A in 2010 concluded a clear relationship between the contagious dose of the virus and patient outcomes.

“Low dose exposure may lead to infection, due to the high infectivity of the virus, but of those infected only a small proportion may become ill,” said the authors. In contrast, “exposure to high doses of virus results in most of the infected subjects also becoming ill.”

One reflective study of the 1918 Spanish Flu finds the much higher mortalities seen in its second and third waves can be enlightened entirely by people being exposed to a more massive contagious dose rather than an alteration in the virus, as has been assumed previously.

Speaking about practical measures, it should probably be steered by the principle of keeping communications fleeting instead of intense. As the government wants, this means that people have greater confidence in returning to the office and other well-ventilated environments, believing they are well set apart.

At home, the family members probably have to distance more, especially if someone carries the virus. One should not share a bed with a contagious partner or child and, as suggested in May, “isolate an infected household member as far as possible.”

In more crowded environments, masks and handwashing are the obvious prevention. As the study mentioned, prophetically predicted when it was published in 2010: “When exposure to airborne virus is reduced, for instance by population-wide use of face masks, the relative decrease in numbers of illnesses is expected to be greater than the relative decrease in [viral] transmission.”

Finally, avoid packed bars and any other crowded areas. The virus’s infection dose will be just as high in that area as it has ever been.

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