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Catching COVID outside rare but not impossible

2nd infection brings more severe symptoms

WASHINGTON (AFP) — Almost all documented coronavirus transmissions have occurred indoors, but experts say that wearing a mask outside is justified because there is still a risk of infection.

The likelihood of catching the virus increases at events where people stand near each other and talk for long periods of time, such as parties or election campaign rallies.

Relative risk

Since the start of the pandemic, studies have described cases of infection in restaurants, houses, factories, offices, conferences, trains and planes.

One study published in April identified a single case of transmission outdoors, between two Chinese villagers, out of more than 7,000 studies.

In an analysis of 25,000 cases, which has not yet been independently reviewed, six percent of cases were linked to environments with an outdoor element, such as sporting events or concerts.

These were enclosed areas where social distancing was not observed, or where people stayed for a while, moving around and talking loudly or singing.

“There were virtually no cases that we could identify that took place in sort of everyday life outdoors,” study author Mike Weed, a professor and researcher at Canterbury Christ Church University, told AFP.

The data indicates that “outdoors is far safer than indoors, for the same activity and distance,” according to a group of scientists and engineers, including professors from American, British and German universities.

‘Dilution through the atmosphere’

“The risk of transmission is much lower outside than inside because viruses that are released into the air can rapidly become diluted through the atmosphere,” the group explained, comparing the virus-carrying “aerosols” to cigarette smoke.

Since February, multiple studies and health authorities have pointed to the airborne path of transmission, by invisible clouds of microscopic droplets (aerosols) that we release by breathing, talking and singing.

This is in addition to the relatively larger droplets that we expel by coughing or sneezing, which can land directly on someone else’s face within a perimeter of one or two meters (up to six feet).

The smallest droplets float in the air for minutes or hours, depending on an area’s ventilation. In a poorly ventilated room, but also outside between two buildings with no air circulation, the droplets can accumulate and get inhaled by a passerby.

The dose of viral particles needed to cause an infection is unknown, but the larger the dose, “the greater the probability of infection,” Steve Elledge, a Harvard University geneticist and expert in viruses, told AFP.

The time spent near a contagious person will be a key factor: a second on the sidewalk doesn’t seem to be enough to catch Covid-19. It probably takes at least several minutes.

“While it is not impossible, there is no evidence that Covid-19 has been transmitted when people walk past each other outdoors,” the group of scientists concluded.

‘Precautionary principle’

Linsey Marr, a well-known expert on airborne virus transmission from Virginia Tech, told AFP that she recommends wearing masks outside if the area is crowded and “you will be passing by people frequently, say, more than one per minute as a guideline but not an absolute rule.”

“When we walk by people outside, we might catch a whiff of their exhaled breath plume,” she said. “Any single brief, passing exposure is low risk, but such exposures might add up over time.”

“My advice follows the precautionary principle and the fact that wearing a mask does not harm,” Marr added.

On restaurant patios, the group of scientists recommends keeping a safe distance between tables and wearing masks while not eating.

There are too many variables to calculate the exact risk on a sidewalk or in a park — it depends on the wind and the number of people but also the sun.

Ultraviolet rays deactivate the virus, but the speed at which they do so depends on the sun’s intensity (from a few minutes to an hour).

Knowledge is limited because scientists have difficulty measuring virus concentrations outdoors, and conducting experiments like they do in laboratory settings.

In terms of public health, experts believe that it is ultimately more efficient to have simple and clear guidelines.

“Having a universal agreement of continued use of mask is really the safest strategy,” said Kristal Pollitt, a professor of epidemiology and environmental engineering at Yale University.

Not to mention that on a sidewalk, a passerby can sneeze the instant you walk by, she told AFP.

More severe symptoms

COVID-19 patients may experience more severe symptoms the second time they are infected, according to research released Tuesday confirming it is possible to catch the potentially deadly disease more than once.

A study published in The Lancet Infectious Diseases journal charts the first confirmed case of Covid-19 reinfection in the United States — the country worst hit by the pandemic — and indicates that exposure to the virus may not guarantee future immunity.

The patient, a 25-year-old Nevada man, was infected with two distinct variants of SARS-CoV-2, the virus that causes COVID-19, within a 48-day time frame.

The second infection was more severe than the first, resulting in the patient being hospitalised with oxygen support.

The paper noted four other cases of reinfection confirmed globally, with one patient each in Belgium, the Netherlands, Hong Kong and Ecuador.

Experts said the prospect of reinfection could have a profound impact on how the world battles through the pandemic.

In particular, it could influence the hunt for a vaccine — the currently Holy Grail of pharmaceutical research.

“The possibility of reinfections could have significant implications for our understanding of COVID-19 immunity, especially in the absence of an effective vaccine,” said Mark Pandori, for the Nevada State Public Health Laboratory and lead study author.

“We need more research to understand how long immunity may last for people exposed to SARS-CoV-2 and why some of these second infections, while rare, are presenting as more severe.”

Waning immunity?

Vaccines work by triggering the body’s natural immune response to a certain pathogen, arming it with antibodies it to fight off future waves of infection.

But it is not at all clear how long COVID-19 antibodies last.

For some diseases, such as measles, infection confers lifelong immunity. For other pathogens, immunity may be fleeting at best.

The authors said the US patient could have been exposed to a very high dose of the virus the second time around, triggering a more acute reaction.

Alternatively, it may have been a more virulent strain of the virus.

Another hypothesis is a mechanism known as antibody dependent enhancement — that is, when antibodies actually make subsequent infections worse, such as with dengue fever.

The researchers pointed out that reinfection of any kind remains rare, with only a handful of confirmed cases out of tens of millions of COVID-19 infections globally.

However, since many cases are asymptomatic and therefore unlikely to have tested positive initially, it may be impossible to know if a given COVID-19 case is the first or second infection.

In a linked comment to The Lancet paper, Akiko Iwasaka, a professor of Immunobiology and Molecular, Cellular and Developmental Biology at Yale University, said the findings could impact public health measures.

“As more cases of reinfection surface, the scientific community will have the opportunity to understand better the correlates of protection and how frequently natural infections with SARS-CoV-2 induce that level of immunity,” she said.

“This information is key to understanding which vaccines are capable of crossing that threshold to confer individual and herd immunity,” added Iwasaka, who was not involved in the study.