QUESTION: I have read that the COVID-19 Delta variant is as contagious as chickenpox. What exactly does that mean?
ANSWER: The medical community measures the infectiousness of a virus by its reproduction number. That’s the average number of new infections that result from each case.
The version of SARS-CoV-2, the coronavirus that causes COVID-19, that was first detected in Wuhan, China, had a reproduction number in the range of two to three. The Delta variant’s is between six and nine. In other words, on average, anyone who catches the virus can be expected to spread it to between six and nine people in the absence of any control measures.
“That’s a game changer,” says Janine McCready, an infectious diseases physician at Michael Garron Hospital in Toronto.
She notes that it is far more contagious than the common cold and the annual flu. And it is approaching the infectious level of chickenpox, which has a reproduction number in the range of nine to 12.
Technically speaking, the Delta variant is not quite as transmissible as chickenpox but the comparison serves as a useful reference point for the general public. People who grew up before routine inoculations against the virus may remember “the explosive nature of chickenpox outbreaks,” says McCready.
The basic reproduction number expresses a property of the virus – how well it can jump from person to person. But human behaviour such as physical distancing, wearing masks and getting vaccinated can thwart the virus from reaching its potential, says Andrew Morris, an infectious diseases specialist at Sinai Health and the University Health Network in Toronto.
However, he adds that the wrong types of behaviour will do the opposite and fuel viral transmission. When a crowd gathers in a poorly ventilated indoor space, an infected individual could spread the Delta variant to far more than six or nine people, leading to a “super-spreader” event.
For these reasons, the factors determining the effective reproduction number “can be quite complicated,” says Morris.
Nonetheless, experts resoundingly agree that the Delta variant poses a far greater threat to public health than earlier versions of this coronavirus.
Research suggests the Delta variant has acquired genetic mutations that boost the “viral load” – the density of the viral particles in the body, including the respiratory tract.
“One study found that the viral load is up to 1,000-times higher in those with the Delta variant,” says Nelson Lee, a professor at the Dalla Lana School of Public Health at the University of Toronto.
Scientists still have a lot to learn about COVID-19. For instance, they don’t yet know the infectious dose – or how many viral particles are needed to cause disease. But there is one thing they can safely assume: a larger dose is more infectious than a smaller one.
“People with the Delta variant are putting out much more virus into the environment through their respiratory droplets and aerosols,” says McCready. “So, those around them are going to be exposed to more virus, increasing their likelihood of becoming infected.”
Other research indicates that the Delta variant has also acquired another mutation that gives it a competitive edge, notes Lee. “There is some evidence to show it actually grows faster in cells,” he adds. “That means people might begin spreading the virus earlier after they become infected.”
With the original strain, it usually took about six or seven days for symptoms to develop and for the person to become infectious. But, in the case of the Delta variant, that time frame shrinks to just four days.
The faster incubation period creates additional challenges for containing the spread of COVID-19. “By the time you realize you have been infected, you could have already exposed a large group of people to the virus,” says McCready.
Another concern is so-called breakthrough infections in which some fully vaccinated people have caught the Delta variant. But those people tend to experience only mild symptoms or no symptoms at all – and they are infectious for only a relatively brief period of time. Essentially, the immune system in a vaccinated person quickly brings the infection under control. Most importantly, they don’t usually end up in the hospital – or worse, dead.
“The protection provided by vaccines is still pretty high,” says Lee. “The fact that they can prevent severe disease means vaccines are doing their job.”
And yet with the Delta variant spreading across the country and around the world, infectious diseases experts say this is not the time to let down our guard.
“You want to do everything you can to minimize your potential exposure,” says McCready. “I think it’s good practice to wear a mask in public settings even if you are vaccinated.”
She adds that people should also try to avoid congested, poorly ventilated indoor spaces where tiny infectious particles may linger in the air.
“Ideally, you want both good ventilation and filtration,” says Morris. “Ventilation primarily dilutes the concentration of aerosols, while filtration actually removes them from the air.”
McCready urges people to “choose wisely” when interacting with people from outside their household.
“So, when you are making your choices, do you want to hang out with your friends outdoors in a park, or do you want to go to a crowded indoor venue?” she asks. “Certainly, I know what I would do to reduce my exposure to unnecessary risks.”
But the best thing we can do to protect ourselves against the Delta variant is to just get vaccinated.
“This virus is highly contagious and it’s spreading fast,” says Morris. “It will find the people who are unvaccinated – especially those who will not or cannot adhere to public health measures.”