First observed in September 2020 in the Kent region of south-east England, the VoC variant 202012/01 (Variant of Concern, year 2020, month 12, variant 01) of the coronavirus is likely to be causing an outbreak of cases in the UK and more recently in Ireland, due to increased contagiousness.
Although definitive proof of a greater transmissibility of this new variant has not yet been made, many clues converge towards this hypothesis. Preliminary work by a team from the London School of Hygiene and Tropical Medicine (LSHTM) concluded that this declination of the virus was approximately 50-74% more contagious (median: 56%) than the majority of SARS-CoV- 2 in circulation since the start of the pandemic. A mutation found in the virus’s spike protein (N501Y) would significantly increase the affinity, and therefore the chemical bond, between the virus and the human cells it can penetrate, which would promote infection.
Contagiousness, a key factor in the dangerousness of the virus
One of the first concerns was whether this new variant was more lethal than the others, that is, if it caused more severe cases of Covid-19 and a greater number of deaths for an equal amount of people. infected.
Spontaneously, the fears center on the risk that the variant could be very fatal. But paradoxically, a more contagious virus can do significantly more damage than a more deadly virus.
Let us imagine a hypothetical mutation of SARS-CoV-2 which would have a case fatality rate 50% higher than that observed for “initial” SARS-CoV-2. The number of deaths, with equal viral circulation, would logically also be increased by 50%. But this increase would be constant over time.
On the other hand, a higher contagiousness will have more serious effects, because it is not a single increase, but a multiple one. Each infected person will in turn infect more people. If we estimate, for example, that the generation time (i.e. the fact of infecting a person after being exposed to the virus) is on average six days, in thirty days a 50% increase in contagiousness will apply at least five times (30/6 = 5): with five “generations” of contamination, so five times an increase of 50%, growth is exponential.
To illustrate this reasoning, British epidemiologist Adam Kucharski took the example on Twitter an initial situation of 10,000 cases of Covid-19:
- by applying a fatality rate of 0.8% and a reproduction rate of the virus of 1.1, the “usual” coronavirus would cause, after thirty days of free propagation, 129 deaths;
- by increasing the death rate by 50%, the number of deaths would be 193 after the same period;
- by increasing contagiousness by 50%, the number of cases would grow much faster, and cause… 978 deaths.
The graph below, the calculations of which were carried out using parameters known in France, illustrates the difference in dangerousness between the British variant (with estimates of its contagiousness) and a theoretically more lethal variant.
If the high contagiousness of this new variant is confirmed, it will most likely be more difficult to control than the spring 2020 virus, and less sensitive to public restriction measures taken for a little less than a year.
The situation in the United Kingdom and Ireland now shows an impressive acceleration in the circulation of the new variant. In Ireland, where the share of the VoC 202012/01 variant rose from 8.6% to 24.9% of the samples sequenced between December 20 and January 3, the number of cases per million inhabitants increased tenfold in just two and a half weeks.
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