Editorial: The three Cs of Covid transmission

People in managed isolation at the Pullman Hotel. Photo / Michael Craig

Opinion

EDITORIAL

It was an unfortunate choice of words this week from director general of health Dr Ashley Bloomfield to describe the official view on the Northland woman who'd tested positive after leaving managed isolation: "We're not breathing out just yet."

It is now well-known that the virus can spread from an infected person's mouth or nose in small liquid particles when they cough, sneeze, speak, sing or heavily exhale. These liquid particles are different sizes, ranging from larger "respiratory droplets" to smaller "aerosols".

Current World Health Organisation advice is that transmission can occur more easily in the "three Cs": crowded places with many people nearby; close-contact settings, especially where people have conversations very near each other; and confined and enclosed spaces with poor ventilation.

The risk of Covid-19 spreading is higher in places where these 3Cs overlap.

It does appear New Zealand has once again - touch wood - avoided a massive outbreak due to a serendipidous separation of 3Cs.

Professor Hussam Jouhara, at Brunel University London, has pointed out in a recent study high occupancy buildings are the highest risk situations for Covid spread as "the most commonly used ventilation systems are inadequate at lowering airborne transmission risks".

Depending on the weather outside, most ventilation systems suck in outdoor air, then heat or cool it before circulating it around the building. The "used" air then either gets pumped back outside or is recirculated in the system.

Most current systems use centralised air distribution and ceiling-level air supply or recirculation, which create the best conditions for disease to spread, the study says. And with claims the virus can stay in the air for up to three hours, the more people come and go from the building, the more the virus spreading pathogens people are exposed to.

Swapping these centralised systems for displacement ventilation systems such as natural ventilation, or naturally assisted ventilation (mixing mechanical extract with controlled inlet openings).

Poor maintenance or alterations to cut energy use or noise also sometimes result in a gap between ventilation levels in building standards and reality, Jouhara's team found, and peak CO2 concentration often exceeded recommended levels.

When considering the list of locations our three positive cases visited before being sequestered into self-imposed isolation, it is interesting to note how few of these fit the bill of the most dangerous places for transmission.

Perhaps it would be timely to remind all those emerging from our managed isolation and quarantine (MIQ) facilities to avoid such places for the ensuing days or weeks (we haven't missed the irony that it is almost precisely describes the very facilities we are using for MIQ).

Again, it is noteworthy that some New Zealand arrivals are coming into the country to attend weddings, funerals or other large gatherings. Yet, our largest clusters have bloomed from a Queenstown cattle conference; an Auckland secondary school; a Bluff wedding; a Matamata St Patrick's celebration; a Christchurch aged care facility; and a Mt Roskill church.

These are the kinds of events and places arrivals should be strongly advised - in the kindest of terms, of course - to avoid for a longer period of time.