By now, the Government’s slogan that Covid-19 is a “tricky virus” has become something of a cliche.
Whenever an infection slips through the border, Jacinda Ardern, Chris Hipkins and Ashley Bloomfield can be relied on to blame the deceptive nature of our viral foe. But what exactly is it about SARS-CoV-2 – the virus that causes the Covid-19 disease – that makes it so tricky?
We’re still learning more about the virus every day, experts say. So far, however, its ability to be transmitted at a distance by people who have yet to show symptoms, combined with an uneven transmission dynamic, means policymakers are presented with serious challenges in reining in Covid-19.
One of the fundamental principles of epidemiology is reducing the effective reproduction number. Also known as the R value, the reproduction number is a representation of the average number of people a Covid-19 case will infect.
In New Zealand at Level 1, modellers have found the R number is around two or 2.5. Every case of Covid-19 in the community in this scenario will go on to infect two other people – on average.
In reality, however, a majority of new cases infect no one and only a small minority are responsible for the bulk of onward spread. This is a pattern that has been observed overseas, but researchers and modellers have established it occurred in New Zealand as well.
Jemma Geoghegan, an evolutionary virologist at the University of Otago, told Newsroom her genomic research showed that some 57 percent of cases that crossed New Zealand’s borders in the original March outbreak passed the virus on to no one. An additional quarter of introductions infected just one other person, while just 19 percent of the cases with overseas origin were responsible for two or more other cases.
“That means that few of the cases are responsible for most of the spread,” she said.
Modelling by Te Pūnaha Matatini came to a similar conclusion for transmission in various clusters during March, finding that one in five cases were responsible for between 65 and 85 percent of onward spread.
This asymmetric distribution of transmission is a result of super-spreading – though whether that spreading occurs due to environmental factors, factors of the people infected or something unique to the virus remains unclear.
“That makes it really tricky,” University of Auckland microbiologist Siouxsie Wiles said.
“There’s obvious things that are features around some of those super-spreading clusters. So being in confined spaces, badly ventilated, with lots of people singing, shouting, things like that. But that might not be the only feature. The problem is you can’t really predict whether somebody there is going to result in one of these super-spreading events.”
This explains why the Northland case, for example, failed to infect her husband while being in the car with him for an extended period of time – even as other people have been infected via bin lids or lift buttons.
That means every case of Covid-19 needs to be treated as a potential super-spreader, even if the majority of them won’t infect anybody.
A second “tricky” aspect of SARS-CoV-2 is the fact that it can be transmitted at longer distances than many people assume.
The standard distancing rules, around staying one to two metres away from others, cover what experts say is the most common mode of transmission: droplets. When speaking, sneezing, coughing or singing, we all spread small droplets from our mouths (and sometimes noses) which can be heavily populated with virus particles. Maintaining distance and wearing a mask can reduce that risk of transmission to near zero, however.
While these principles were well-established near the start of the pandemic, more recent research has pointed to transmission via much smaller droplets – called aerosol transmission – as a potent secondary vector for infection. Aerosols can travel much farther than a couple of metres and can also be picked up and redistributed by ventilation systems.
“It’s becoming quite clear that aerosol and droplet transmission is the primary mechanism for transmission, and there’s a lot of evidence for that,” Geoghegan said.
Wiles added: “There’s all sorts of really good studies now showing this aerosol transmission – showing it’s going through piping and ventilation ducts.”
Numerous case studies since the start of the pandemic have found aerosol transmission playing a larger role in infection than previously expected. One team of researchers now estimates airborne transmission was responsible for more than half of the transmission identified on the Diamond Princess cruise ship in February 2020.
In a restaurant in Guangzhou, China, in January 2020, an air conditioner helped facilitate aerosol spread from one person to five others who didn’t have face-to-face interactions with the index case. Four people dining with the index case were also infected.
A study of a cluster in a department store in China, also from the early stages of the pandemic, identified that 12 of 42 people infected in the store received the virus via aerosol. Just five were infected by close contact. The transmission vector for the remainder was unclear, researchers reported.
The study concluded that SARS-CoV-2 “could travel considerable distance (over several meters) by aerosol transmission”.
In addition to long-distance transmission, aerosols can render specific locations unsafe, Wiles said.
“There’s this idea of shared air. We don’t know how long it might linger around – that may well depend on ventilation. If somebody’s only been in a place for a few minutes and then leaves, but they were infectious, how long afterwards are people at risk from that place?”
The CDC advises that aerosol transmission is most likely to occur indoors, when someone has been exposed to aerosols for a long time, when the density of aerosols is high (because of shouting, singing or exercising) or where ventilation fails to keep air circulating out of the space.
Wiles sees asymptomatic and pre-symptomatic transmission as another difficult-to-manage aspect.
“The [other] tricky thing is that people are infectious before they realise it,” she said.
“It’s hard to stop transmission before it’s happened and it’s why contact tracing in those countries that can do the contact tracing is so important. Because by putting this ring around the case where anybody who they’ve been in contact with has been exposed, you can basically isolate them to stop any further transmission.”
In sum, Covid-19 can be spread at long distances by people who don’t know they have it, and while most people who have it won’t spread it, a minority have the potential to be super-spreaders.
“That’s why it’s a tricky virus. We’re still learning so much about this virus and there are still clearly gaps in our knowledge,” Geoghegan said.
The new variants throw another wrench in the works, as they could change how the virus operates on a fundamental level.
“The other thing we don’t know is whether there’s going to be any changes in these kinds of things, like incubation periods or infectious periods or symptoms, with the new variant. There’s been reports from some places that maybe there’s more gastrointestinal symptoms with them. Who’s going to want to go for a nasal swab every time they’ve got a runny tummy?” Wiles said.
“With the new variants, it looks like there may well be a need to do more than one ring of contact tracing because some people are becoming infectious so quickly that they’ve already passed it on by the time the index case has been picked up.”
Geoghegan agreed. “Although it seems like it’s been a long time, we’ve only actually known about this virus for just over a year, so we’re still learning a lot,” she said.
Given all of this, Wiles says its best to err on the side of caution with more stringent measures. In New Zealand, with such low prevalence of Covid-19, we have to protect not just against the most common methods of transmission but also against the edge cases, which have just as much potential to thrust us back into lockdown.
“It just shows how hard it is to control without using quite strict public health measures like we’ve put in place.”