“Our goal must be low case numbers”

Tanja Stadler, ETH Professor of Computational Evolution, is working at the forefront in the fight against the coronavirus. She recently became head of the Swiss National COVID-19 Science Task Force’s Data and Modelling Group, which closely studies the new variants of SARS-CoV-2.
Tanja Stadler recently became head of the data and modelling group in the Swiss National COVID-​19 Science Task Force. (Image: Giulia Marthaler/ETH Zurich)

How rapidly are the two new variants from South Africa and the UK currently spreading in Switzerland?
The variant 501Y.V2 that first appeared in South Africa has been identified only in a small number of samples. B 1.1.7 – the variant from the UK – accounted for about 10 percent of all confirmed cases in Switzerland in the third week of January. In certain regions, there are indications of an even higher proportion.

Does this worry you?
The situation is in line with what we expect based on data from the UK. The relative frequency of the variant B 1.1.7 is increasing in Switzerland due to its higher transmission rate. The current big question is: is the absolute number of B 1.1.7 infections also increasing? We are currently seeing the numbers doubling approximately once a week – from around 700 cases in the second week of January to around 1,300 in the third. In the next few days, the data will tell us whether the new measures which were implemented on 18 January had the desired effect.

How do the two variants affect the R-value? Are they causing it to shoot up?
As the proportion of infections caused by the new variant is currently about 10 percent, the R-value is still dominated by the other variants. But when infections by the new variants reach the mid-double-digits, we will see a significant impact on the R-value.

And when will this happen?
We anticipate that B 1.1.7 will be the dominant variant in March.

«Lower case numbers mean that there is less virus circulating and so SARS-CoV-2 has less chance of developing variants that make the vaccine less effective»      Tanja Stadler

Do we have any chance of slowing down the two variants?
For B 1.1.7, data suggests that this variant is more transmissible. Without targeted measures such as intensive contact tracing in cases attributable to B 1.1.7, the relatively frequency of this variant will continue to increase. We must be realistic, however: targeted contact tracing over the Christmas holidays, at a time when few cases were known, was unable to stop the spread.

Are there any other variants which are of concern to you?
Yes, unfortunately: 501Y.V3 was identified in Brazil. This variant is spreading rapidly through areas of Brazil where many people had been previously infected. Laboratory analysis indicates that the immune response of re-infected people may be weakened. This would mean that the risk of reinfection by SARS-CoV-2 may be increased.

Why are we suddenly seeing variants possibly altering the epidemiology of the virus?
Variants always occur – the virus mutates every two weeks on average. And the more viruses circulate worldwide, the more variants emerge. What is of interest to us is the question of whether variants with new characteristics emerge. We are particularly interested in variants with a different likelihood of transmission, those with a different disease progression and those that weaken an individual’s immune response following a previous infection or vaccination.

What is the role of monitoring in this context?
It is absolutely central, as it is the only way to identify new variants rapidly. It is particularly important in the vaccination phase: we have to be extremely quick to identify variants for which the efficacy of vaccination could be reduced. Between March and November 2020, we sequenced about 100 samples every week, which corresponded to three to seven percent of all cases in summer. During the second wave, however, the number of new infections rose so rapidly that this percentage fell accordingly. Since the Christmas holiday period, we have increased our capacity to about 1,000 samples per week. The staff at ETH have been fantastic, working at top speed over the holidays both in the lab and on their laptops!

Does vaccination have an impact on the emergence of new variants?
Any intervention – of which vaccination is one – places further selection pressure on the virus. We see this with seasonal influenza, for example, which escapes our immune response every year. Or with bacteria, as they become increasingly resistant to antibiotics.  SARS-CoV-2 will also evolve further.

Case numbers are currently falling. What does this mean for vaccination?
Low case numbers have many advantages – I can't list them all here, but I will answer the question in regard to vaccination. Healthcare professionals are less busy with Covid cases, are themselves healthy and thus able to work on vaccination campaigns. A large proportion of the population is healthy and can be vaccinated. Sick people cannot be vaccinated. And lower case numbers mean that fewer viruses are circulating, giving SARS-CoV-2 less opportunity to develop variants that affect the efficacy of the vaccination. Our goal must be low case numbers.