How much more contagious are the variants of Covid-19? The dangers of a more communicable disease, explained.


A more contagious SARS-CoV-2 virus is more deadly. Not because it makes an individual sicker, but because it affects more people. More disease means more cases: more mild, more serious cases, more long-term complications, more hospitalizations and more deaths.

There is not a perfect estimate of the contagion of the SARS-CoV-2 variant called B.1.1.7, which was first discovered in the UK. More careful lab work needs to be done on the animals to nail it down, which could take a few more weeks. But according to preliminary estimates, it is between 30% and 70% more contagious. This variant is now appearing around the world: in the United States, throughout Europe, in Asia and elsewhere. This is of particular concern, given that in the United States and elsewhere, the spread of the pandemic is still reaching new heights.

There is also a variant called 501Y.V2 which was first found in South Africa; it appears to be less widespread than B.1.1.7, but also more contagious than the older variants. It is always possible that other more transmissible variants will emerge in the future. Scientists just don’t know.

Regardless of the degree of transmissibility of a new variant, any possible increase in transmissibility is of great concern to epidemiologists and other public health actors. This means now, more than ever, that our collective and individual actions to stop the spread are of critical importance.

Here are some main reasons.

A more transmissible virus just means more cases

The reason for concern about a more contagious variant of the virus is simple. “Once [the variant] becomes common, it will dramatically speed up transmission, ”Harvard epidemiologist Marc Lipsitch said during a press call. Expedited transmission means more cases.

This is because the virus is growing exponentially. Any increase in its inherent transmissibility leads to an increasing number of cases down the line, if control measures do not also accelerate to keep pace.

With a 50 percent increase in contagiousness, “In less than two weeks, you get twice as many cases,” Lipsitch says. “And in a month or so, you have four, five times as many cases. But it is very approximate. It could be higher, he said.

A more transmissible virus means we have to work harder to stop the spread

Epidemiologists think of the transmissibility of a virus with a number called R, or the basic reproduction number. This describes how many new cases, on average, will follow a case of the virus.

At the start of the epidemic, before the world kicked in, epidemiologists estimated the R to be around 2 or 3. Since then, thanks to our efforts (implemented inconsistently) – mask wearing, social distancing , business closures, etc. R has been reduced in many states to around 1.1 to 1.2. But as long as the R-number is greater than 1, the virus can spread exponentially.

Let’s say a community has reduced its R-value to 1. Then a 50% more transmissible variant happens. This could mean that the same mitigation effort that brought the old variant of the virus down to 1 now only drops it down to 1.5.

That is: to fight a more transmissible version of the virus, communities need even tighter controls.

50% more transmissible virus, says Lipsitch, means “we need to reduce our contacts by another third from the already strong restrictions [already in place] in order to come back to the same place where we were. This could mean the closure of businesses that have partially reopened, the closure of schools and other foreclosure measures.

If communities do nothing in the face of a more contagious variant, the numbers could add up very quickly. Bill Hanage, another Harvard epidemiologist, walked me through the following thinking experience.

Let’s say a community has the virus more or less under control. The R value in the zone is 1, which means that the number of cases in the zone remains constant from month to month.

In this scenario, “you have 1,000 cases now, and you will have 1,000 a month,” says Hanage (his scenario assumes that it takes about five days for one infection to cause another). “Now imagine that the virus is 50% more transmissible. …. 1,000 cases of such a virus would now translate into over 10,000 cases per month if you did nothing! “

Christina Animashaun / Vox

More transmissible virus increases number of people who need to be vaccinated

A more transmissible virus also increases the herd immunity threshold, or the rough estimate of the percentage of people in a population who need to gain immunity, ideally through vaccination, for the epidemic to shrink in size.

The collective immunity thresholds depend on the value of R. The higher the value R, the higher the threshold. If a more transmissible variant becomes dominant in the world, it increases the pressure on vaccination campaigns to immunize even more people. Already, it appears that more than 70% of the population will need to be immunized to achieve collective immunity (although the exact number needed is unknown). This is already an extremely high bar and difficult to cross, given the slow pace of early vaccine distribution.

There is also the still unclear possibility that the virus could mutate into a form that reduces the effectiveness of current vaccines.

A more transmissible virus means more deaths

Good news about the B.1.1.7 variant: so far, it doesn’t appear to be more deadly than the older variants of the virus. In other words: anyone who suffers from it is no longer likely to end up in hospital or die, according to initial data.

But more infections mean more deaths. “In general, the more people are infected, the more the number of hospitalizations and deaths increases in proportion to this number,” explains Emma Hodcroft, molecular epidemiologist at the Institute of Social and Preventive Medicine in Switzerland. “So more cases is bad news as well.”

A virus 50% more contagious is even more deadly than a virus 50% more deadly, according to epidemiologist at the London School of Hygiene & Tropical Medicine Adam Kucharski explains on Twitter.

If a city has an R-value of 1.1 and there are 10,000 people infected, in a month you would expect to see 129 deaths, he says.

Increase virus mortality by 50% in this scenario and you would expect 193 deaths, an increase of 49.6%.

Increase the contagiousness of the virus in this scenario by 50% and you get 978 deaths, Kucharski notes. That’s an increase of 658%. It’s a theoretical calculation, but the central point is the exponential effect, as Kucharski explains.

It’s not entirely clear if this will happen

Again, the estimate of an increased transmissibility of 50% is only an estimate. And even though the newer variants are more transmissible, it might be difficult to predict just how more transmissible they will be in different places.

Hanage points out that the reported 50% more transmissible figure is based on the behavior of this variant in the UK. In a different location – with different social distancing adhesions, different mitigation measures in place – the new variant might behave differently. It’s complicated, yes, but these are epidemics for you.

But even the hypothetical threat of a more communicable variant is a call to action.

To prevent more mutations from occurring, to prevent the emergence of potentially problematic variants, we need to stop the spread of SARS-CoV-2 in general. On the one hand, it helps us deal with the pandemic as a whole. But “it’s also in a practical way that we get fewer emerging variants,” Angela Rasmussen, a virologist at the Center for Global Health Science and Security in Georgetown, told me in an interview in December. “If the virus does not replicate, it cannot mutate. And if it cannot mutate, new variants cannot emerge. “

The new variant means that our individual actions matter more than ever. This variant spreads in the same way as other variants of SARS-CoV-2, and the same precautions apply. Social distancing, mask wear and ventilation are more important than ever.

With the new variant, Lipsitch says, “Your isolation, quarantine, and contact tracing are significantly more important, even more so than ever before. So you are doing more for the world. “

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