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The coronavirus can stay aloft for hours in tiny droplets in stagnant air, infecting people as they inhale, mounting scientific evidence suggests.

This risk is highest in crowded indoor spaces with poor ventilation, and may help explain superspreading events reported in meatpacking plants, churches and restaurants.

It’s unclear how often the virus is spread via these tiny droplets, or aerosols, compared with larger droplets that are expelled when a sick person coughs or sneezes, or transmitted through contact with contaminated surfaces, said Linsey Marr, an aerosol expert at Virginia Tech.

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Aerosols are released even when a person without symptoms exhales, talks or sings, according to Marr and more than 200 other experts, who have outlined the evidence in an open letter to the World Health Organization.

What is clear, they said, is that people should consider minimizing time indoors with people outside their families. Schools, nursing homes and businesses should consider adding powerful new air filters and ultraviolet lights that can kill airborne viruses.

What does it mean for a virus to be airborne?

For a virus to be airborne means that it can be carried through the air in a viable form. For most pathogens, this is a yes-no scenario. HIV, too delicate to survive outside the body, is not airborne. Measles is airborne, and dangerously so: It can survive in the air for up to two hours.

For the coronavirus, the definition has been more complicated. Experts agree that the virus does not travel long distances or remain viable outdoors. But evidence suggests it can traverse the length of a room and, in one set of experimental conditions, remain viable for perhaps three hours.

How are aerosols different from droplets?

Aerosols are droplets, droplets are aerosols — they do not differ except in size. Scientists sometimes refer to droplets fewer than 5 microns in diameter as aerosols. (By comparison, a red blood cell is about 5 microns in diameter; a human hair is about 50 microns wide.)

From the start of the pandemic, the WHO and other public health organizations have focused on the virus’s ability to spread through large droplets that are expelled when a symptomatic person coughs or sneezes.

These droplets are heavy, relatively speaking, and fall quickly to the floor or onto a surface that others might touch. This is why public health agencies have recommended maintaining a distance of at least 6 feet from others, and frequent hand washing.

But some experts have said for months that infected people also are releasing aerosols when they cough and sneeze. More important, they expel aerosols even when they breathe, talk or sing, especially with some exertion.

Scientists know now that people can spread the virus even in the absence of symptoms — without coughing or sneezing — and aerosols might explain that phenomenon.

Because aerosols are smaller, they contain much less virus than droplets do. But because they are lighter, they can linger in the air for hours, especially in the absence of fresh air. In a crowded indoor space, a single infected person can release enough aerosolized virus over time to infect many people, perhaps seeding a superspreader event.

For droplets to be responsible for that kind of spread, a single person would have to be within a few feet of all the other people, or to have contaminated an object that everyone else touched. All that seems unlikely to many experts: “I have to do too many mental gymnastics to explain those other routes of transmission compared to aerosol transmission, which is much simpler,” Marr said.

Can I stop worrying about physical distancing and washing my hands?

Physical distancing is still very important. The closer you are to an infected person, the more aerosols and droplets you may be exposed to. Washing your hands often is still a good idea.

What’s new is that those two things may not be enough. “We should be placing as much emphasis on masks and ventilation as we do with hand washing,” Marr said. “As far as we can tell, this is equally important, if not more important.”

Should I begin wearing a hospital-grade mask indoors? And how long is too long to stay indoors?

Health care workers may all need to wear N95 masks, which filter out most aerosols. At the moment, they are advised to do so only when engaged in certain medical procedures that are thought to produce aerosols.

For the rest of us, cloth face masks will still greatly reduce risk, as long as most people wear them. At home, when you’re with your own family or with roommates you know to be careful, masks are still not necessary. But it is a good idea to wear them in other indoor spaces, experts said.

As for how long is safe, that is frustratingly tough to answer. A lot depends on whether the room is too crowded to allow for a safe distance from others and whether there is fresh air circulating through the room.

What does airborne transmission mean for reopening schools and colleges?

This is a matter of intense debate. Many schools are poorly ventilated and are too poorly funded to invest in new filtration systems. “There is a huge vulnerability to infection transmission via aerosols in schools,” said Don Milton, an aerosol expert at the University of Maryland.

Most children younger than 12 seem to have only mild symptoms, if any, so elementary schools may get by. “So far, we don’t have evidence that elementary schools will be a problem, but the upper grades, I think, would be more likely to be a problem,” Milton said.

College dorms and classrooms are also cause for concern.

Milton said the government should think of long-term solutions for these problems. Having public schools closed “clogs up the whole economy, and it’s a major vulnerability,” he said.

“Until we understand how this is part of our national defense, and fund it appropriately, we’re going to remain extremely vulnerable to these kinds of biological threats.”

What are some things I can do to minimize the risks?

Do as much as you can outdoors. Despite the many photos of people at beaches, even a somewhat crowded beach, especially on a breezy day, is likely to be safer than a pub or an indoor restaurant with recycled air.

But even outdoors, wear a mask if you are likely to be close to others for an extended period.

When indoors, one simple thing people can do is to “open their windows and doors whenever possible,” Marr said. You can also upgrade the filters in your home air-conditioning systems, or adjust the settings to use more outdoor air rather than recirculated air.

Public buildings and businesses may want to invest in air purifiers and ultraviolet lights that can kill the virus. Despite their reputation, elevators may not be a big risk, Milton said, compared with public bathrooms or offices with stagnant air where you may spend a long time.

If none of those things are possible, try to minimize the time you spend in an indoor space, especially without a mask. The longer you spend inside, the greater the dose of virus you might inhale.

May 14: The UN’s children agency has warned that an additional 6,000 children could die daily from preventable causes over the next six months as the COVID-19 pandemic weakens the health systems and disrupts routine services, the first time that the number of children dying before their fifth birthday could increase worldwide in decades.

As the coronavirus outbreak enters its fifth month, the UN Children’s Fund (UNICEF) requested USD 1.6 billion to support its humanitarian response for children impacted by the pandemic.

The health crisis is “quickly becoming a child rights crisis. And without urgent action, a further 6,000 under-fives could die each day,” it said.

With a dramatic increase in the costs of supplies, shipment and care, the agency appeal is up from a USD 651.6 million request made in late March – reflecting the devastating socioeconomic consequences of the disease and families’ rising needs.

"Schools are closed, parents are out of work and families are under strain," UNICEF Executive Director Henrietta Fore said on Tuesday.

 “As we reimagine what a post-COVID world would look like, these funds will help us respond to the crisis, recover from its aftermath, and protect children from its knock-on effects.”

The estimate of the 6,000 additional deaths from preventable causes over the next six months is based on an analysis by researchers from the Johns Hopkins Bloomberg School of Public Health, published on Wednesday in the Lancet Global Health Journal.

UNICEF said it was based on the worst of three scenarios analysing 118 low and middle-income countries, estimating that an additional 1.2 million deaths could occur in just the next six months, due to reductions in routine health coverage, and an increase in so-called child wasting.

Around 56,700 more maternal deaths could also occur in just six months, in addition to the 144,000 likely deaths across the same group of countries. The worst case scenario, of children dying before their fifth birthdays, would represent an increase "for the first time in decades,” Fore said.

"We must not let mothers and children become collateral damage in the fight against the virus. And we must not let decades of progress on reducing preventable child and maternal deaths, be lost,” she said.

Access to essential services, like routine immunisation, has already been compromised for hundreds of millions of children and threatens a significant increase in child mortality.

According to a UNICEF analysis, some 77 per cent of children under the age of 18 worldwide are living in one of 132 countries with COVID-19 movement restrictions.

The UN agency also spotlighted that the mental health and psychosocial impact of restricted movement, school closures and subsequent isolation are likely to intensify already high levels of stress, especially for vulnerable youth.

At the same time, they maintained that children living under restricted movement and socio-economic decline are in greater jeopardy of violence and neglect. Girls and women are at increased risk of sexual and gender-based violence.

The UNICEF pointed out that in many cases, refugee, migrant and internally displaced children are experiencing reduced access to protection and services while being increasingly exposed to xenophobia and discrimination.

“We have seen what the pandemic is doing to countries with developed health systems and we are concerned about what it would do to countries with weaker systems and fewer available resources,” Fore said.

In countries suffering from humanitarian crises, UNICEF is working to prevent transmission and mitigate the collateral impacts on children, women and vulnerable populations – with a special focus on access to health, nutrition, water and sanitation, education and protection.

To date, the UN agency said it has received USD 215 million to support its pandemic response, and additional funding will help build upon already-achieved results.

Within its response, UNICEF has reached more than 1.67 billion people with COVID-19 prevention messaging around hand washing and cough and sneeze hygiene; over 12 million with critical water, sanitation and hygiene supplies; and nearly 80 million children with distance or home-based learning.

The UN agency has also shipped to 52 countries, more than 6.6 million gloves, 1.3 million surgical masks, 428,000 N95 respirators and 34,500 COVID-19 diagnostic tests, among other items.

Beijing:  Around 5,000 people have signed up for the phase I clinical trial of recombinant novel coronavirus vaccine in Chinese city Wuhan where the virus first emerged late last year.

The recruitment for participants ended this week with nearly 5,000 volunteers signing up for the trial, state-run Beijing News reported on Wednesday.

A single-centre, open and dose-escalation phase I clinical trial for recombinant novel coronavirus vaccine (adenoviral vector) will be tested in healthy adults aged between 18 and 60 years, according to the ChiCTR (China Clinical Trial Register).

The trial, led by experts from the Academy of Military Medical Sciences, gained its approval on March 16 and the research is expected to last half a year.

Requiring at least 108 participants, the trial will be conducted in Wuhan, capital of Hubei province, the region worst-affected by the virus in the country, state-run China Daily reported.

Participants will experience 14-day quarantine restrictions after being vaccinated and their health condition will be recorded every day.

Chinese scientists are hastening the development of COVID-19 vaccines through five approaches --- inactivated vaccines, genetic engineering subunit vaccines, adenovirus vector vaccines, nucleic acid vaccines and vaccines using attenuated influenza virus as vectors.

So far, most teams are expected to complete preclinical research in April and some are moving forward faster, Wang Junzhi, an academician with the Chinese Academy of Engineering said.

Wang noted that research and development of COVID-19 vaccines in China is not slower than foreign counterparts and has been carried out in a scientific, standardised and orderly way.

China has stepped up the process to finalise vaccines to counter COVID-19 after Kaiser Permanente research facility in Seattle and Washington stole the march and began human trials.

China lifted tough restrictions on the Hubei province on Wednesday after a months-long lockdown as the country reported no new domestic cases.

But there were another 47 imported infections from overseas, the National Health Commission said. In total, 474 imported infections have been diagnosed in China -- mostly Chinese nationals returning home.