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Research and analysis
Long term evolution of SARS-CoV-2, 26 July 2021
Paper prepared by academics on the viral evolution of COVID-19.
From : SAGE
Scientific Advisory Group For Emergencies
( note: computer modelling )
Details
Paper presented by group of academics on scenarios for the longer term evolution of SARS-CoV-2. It was considered at SAGE 94 on 22 July 2021 and updated on 26 July 2021.
The paper is the assessment of the evidence at the time of writing. As new evidence or data emerges, SAGE updates its advice accordingly.
These documents are released as pre-print publications that have provided the government with rapid evidence during an emergency. These documents have not been peer-reviewed and there is no restriction on authors submitting and publishing this evidence in peer-reviewed journals.
Published 30 July 2021
Can we predict the limits of SARS-CoV-2 variants and their phenotypic consequences?
As eradication of SARS-CoV-2 will be unlikely, we have high confidence in stating that there will always be variants. The number of variants will depend on control measures.
We describe hypothetical scenarios by which SARS-CoV-2 could further evolve and acquire, through mutation, phenotypes of concern, which we assess according to possibility. For this purpose, we consider mutations in the ‘body’ of the virus (the viral genes that are expressed in infected cells and control replication and cell response), that might affect virus fitness and disease severity, separately from mutations in the spike glycoprotein that might affect virus transmission and antibody escape.
We assess which scenarios are the most likely and what impact they might have and consider how these scenarios might be mitigated. We provide supporting information based on the evolution of SARS-CoV-2, human and animal coronaviruses as well as drawing parallels with other viruses.
Scenario One:
A variant that causes severe disease in a greater proportion of the population than has occurred to date. For example, with similar morbidity/mortality to other zoonotic coronaviruses such as SARS-CoV (~10% case fatality) or MERS-CoV (~35% case fatality). This could be caused by:
1. Point mutations or recombination with other host or viral genes. This might occur through a change in SARS-CoV-2 internal genes such as the polymerase proteins or accessory proteins. These genes determine the outcome of infection by affecting the way the virus is sensed by the cell, the speed at which the virus replicates and the anti-viral response of the cell to infection. There is precedent for Coronaviruses (CoVs) to acquire additional genes or sequences from the host, from themselves or from other viruses.
2. By recombination between two VOC or VUIs. One with high drift (change in the spike glycoprotein) from the current spike glycoprotein gene used in the vaccine and the other with a more efficient replication and transmission determined by internal genes, for example, a recombination between beta and alpha or delta variants respectively. Alternatively, recombination may occur between two different variants with two different strategies for overcoming innate immunity, combining to give an additive or synergistic change of phenotype resulting in higher replication of the virus – and potentially increased morbidity and mortality.
Likelihood of genotypic change in internal genes: Likely whilst the circulation of SARS- CoV-2 is high.
Likelihood of increased severity phenotype: Realistic possibility.
Impact: High. Unless there is significant drift in the spike glycoprotein gene sequence, then the current spike glycoprotein-based vaccines are highly likely to continue to provide protection against serious disease. However, an increase in morbidity and mortality would be expected even in the face of vaccination since vaccines do not provide absolute sterilising immunity i.e. they do not fully prevent infection in most individuals.
What can we do?
• Consider vaccine booster doses to maintain protection against severe disease.
• Reduce transmission of SARS-CoV-2 within the UK (to reduce risk of point
mutations, recombination).
• Minimise introduction of new variants from other territories (to reduce risk of
recombination between variants).
• Targeted surveillance for reverse zoonoses, and if necessary, consider animal
vaccination, slaughter, or isolation policies.
• Continue to monitor disease severity associated with variants (to identify
changes in phenotype).
• Continue to develop improved prophylactic and therapeutic drugs for SARS-
CoV-2 and disease symptoms.
• Consider stockpiling prophylactic and therapeutic drugs for SARS-CoV-2.
Scenario Two:
A variant that evades current vaccines. This could be caused by:
3. Antigenic ‘shift’: Natural recombination events that insert a different spike gene sequence (or partial sequence) from human CoVs MERS-CoV (highly unlikely due to the low frequency of MERS-CoV infections), or from currently circulating endemic human CoVs (more likely due to the prevalence of these viruses). This would recombine into the ‘body’ of SARS-CoV-2 that is capable of high replication in human cells. The consequence could be a virus that causes disease at a level similar to COVID-19 when it first emerged but against which our current battery of spike glycoprotein-based vaccines would not work.
Likelihood: Realistic possibility.
Impact: High for a completely new spike, medium/low if a spike from a seasonal CoV is introduced since we expect a proportion of the population to have antibodies to these endemic viruses.
What could we do? In the case of introduction of a completely different spike glycoprotein, a similar vaccine platform could be rapidly employed as has been used successfully on the original Wuhan SARS-CoV-2 and subsequent variants. However, there would be a time lag for roll out whilst these vaccines were generated in sufficient quantities to control and mitigate the effects of infection.
4. A longer-term version of shift whereby SARS-CoV-2 undergoes a reverse zoonotic event into an animal reservoir(s). This virus is then on a separate evolutionary trajectory because the virus animals is subject to different selection processes than in humans. The SARS-CoV-2 decedents then re-emerge into humans at a later time when vaccines that have been updated to keep pace with drift in humans sufficiently mismatched so as not able to provide immunologic cross protection.
Likelihood: Realistic possibility. Impact: Medium.
What could we do? Maintain a capacity to make vaccines with updated/different spike protein variants and begin to develop broader CoV immunity in the human population to diverse coronaviruses. For example, begin to develop a universal coronavirus vaccine with strong cross protection to other CoVs potentially using other viral proteins rather than just the spike glycoprotein.
5. Antigenic drift: A gradual or punctuated accumulation of antigenic variation that eventually leads to current vaccine failure. Worst case is that this drift combines with significant antigenic sin (vaccination resulting in an immune response that is dominated by antibodies to previously experienced viruses/vaccines) meaning that it becomes difficult to revaccinate to induce antibodies to the new strains. Genetic and antigenic drift are almost inevitable. Antigenic sin has not yet been reported for SARS-CoV-2 so we consider this possibility less likely.
Likelihood: Almost certain. Impact: Medium.
What could we do? Need to continue vaccinating vulnerable age groups at regular periods with updated vaccines to the dominant antigenic drift variants to increase an individual’s immunological protection against SARS-CoV-2 variants.
• Monitor antigenic variants and update candidate vaccines to cover antigenic escape variants.
• Conduct clinical trials of re-vaccination with antigenically distant vaccines
• Consider clinical trials of multi-valent vaccines.
• Re-vaccinate vulnerable age groups at regular periods with updated vaccines to
the dominant antigenic drift variants to increase an individual’s immunological
landscape to SARS-CoV-2 variants.
• Reduce transmission of SARS-CoV-2 within the UK (to reduce risk of point
mutations, recombination).
• Minimise introduction of new variants from other territories (to reduce risk of
recombination between variants).
• Monitor for reverse zoonoses and if necessary, consider animal vaccination,
slaughter, or isolation policies.
• Continue to develop improved prophylactic and therapeutic drugs for SARS-
CoV-2.
• Stockpile prophylactic and therapeutic drugs for SARS-CoV-2.
... read all: https://www.gov.uk/government/publications/long-term-evolution-of-sars-cov-2-26-july-2021
