Ventilation ‘key’ for preventing spreading

• Call for greater emphasis on opening windows of schools, buses and taxis, and more consideration of staging gatherings outdoors

As evidence mounts of airborne transmission of Covid-19, two leading scientists advising the government say more attention needs to be paid to ventilation in crowded places and public transport. They suggest greater emphasis should be placed on opening windows in schools, buses and taxis, and more consideration given to holding events such as religious gatherings outdoors.

With mounting evidence of airborne transmission of Covid-19, two leading scientists advising the government say more attention needs to be paid to ventilation in crowded places and public transport.

They suggest greater emphasis on opening windows of schools, buses and taxis, and giving more consideration to holding events such as religious gatherings outdoors.

“The contribution of airborne transmission has been underappreciated. But the evidence is mounting that it is probably an important source of acquisition of the virus. It calls into question what we do now,” said Wits vaccinologist Shabir Madhi, who is a member of health minister Zweli Mkhize’s advisory committee on Covid-19.

Airborne transmission was the most plausible explanation for the many secondary cases detected in health facilities and poorly ventilated settings where large numbers of people had congregated, he said in an interview with Business Day on Wednesday.

Madhi called into question the government decision to permit religious gatherings of up to 50 people. Hand hygiene, washing, social distancing and masks remained important interventions to prevent the spread of the virus, he said.

Medical Research Council president Glenda Gray, who is also an advisory committee member, told MPs that emerging evidence of airborne transmission of Covid-19 meant people should not congregate in closed spaces, and public transport should keep windows open. “We have to make sure we ventilate places where there are crowds and avoid congestion if possible. Maybe even churches should be held in open areas,” she said.

Government guidance, which is aligned with World

Health Organisation (WHO) recommendations, emphasises hand washing, social distancing and wearing cloth masks to reduce the spread of small virus-containing droplets falling quickly to the ground after being exhaled by infected people.

But the government does not specifically caution people against gathering in closed environments where they breathe shared air and may be at risk of airborne transmission.

The WHO has until now said there was insufficient evidence to conclude airborne transmission, in which people are infected by inhaling tiny droplets containing virus particles that linger in the air for minutes or perhaps hours after being breathed out by an infected person, was a threat in closed spaces.

But after 239 scientists from 32 countries published a letter in the journal Clinical Infectious Diseases, calling for it to change its position, the agency acknowledged on Tuesday that this could be a threat.

The WHO said it was reviewing the evidence, and planned to release updated recommendations in the next few days.

The National Institutes for Communicable Diseases’s (NICD’s) specialist pathologist, Kerrigan McCarthy, said scientists had no doubt that SARSCoV-2,

the virus that causes Covid-19, could be transmitted by droplets greater than five microns in diameter (which are coughed, sneezed or breathed out by an infected person), or by contact with infected secretions.

But there had been no animal experiments to provide definitive empirical evidence of airborne transmission, so scientists had to rely on theoretical knowledge of how germs cause disease and observations from outbreaks to propose transmission mechanisms.

McCarthy said that if airborne transmission did occur, it was likely to account for fewer infection events than those caused by droplet transmission or direct contact.

“The vast majority of transmission events take place among close contacts at home, work and in confined spaces where close proximity suggests that droplet transmission is the primary mode,” she said.

Moreover, larger droplets contained more infectious material, and were less susceptible to environmental stressors such as heat or drying, she said.

The importance of ventilation was emphasised in the NICD’s guidelines on infection control and prevention for health-care settings, she said.