Mandatory Day 2 and Day 8 PCR testing and variant sequencing of international arrivals has been recently introduced by the UK Government to mitigate against cross-border transmission of high-risk SARS-CoV-2 variants.

SARS-CoV-2 testing and sequencing combines PCR with Next Generation Sequencing Assay and retrospective analysis of test trending data was performed from initiation of testing on 11th March through to the 14th April 2021.  

During this time interval, 203,065 SARS-CoV-2 PCR tests were performed with 3,855 samples testing positive, giving a prevalence of 1.9%.

In total 1,913 SARS-CoV-2 genomes were sequenced from positive cases with CT values less than 30 and 1,635 sequences passed quality metrics for lineage analysis.  

A high diversity of 49 different SARS-CoV-2 variants were identified including:

  • B.1.1.7 Kent 80.6%
  • B.1.351 South Africa 4.2%
  • B.1.617.2 India; 1.7%
  • P.1 Brazil 0.4% 
  • B.1.1.7 with E484K Bristol 0.2%.  

Vaccine effectiveness was age-related and dose-dependent, ranging from 5% in those over 60 with a single dose to 83% in less than 60 with both doses of vaccine.  

Viral load was variant dependent with the Indian variant the Delta variant showing a 21 fold increase in viral copy number compared to the other variants.

The cross-border transmission of SARS-CoV-2 during resumption of international travel is a public health risk to the United Kingdom and could potentially undermine the success of the ongoing vaccination programme.  

High-risk variants characterised by mutations in the spike protein domain have been shown to reduce neutralisation sensitivity to convalescent sera and monoclonal antibodies and therefore have the potential to bypass the protection afforded by the current vaccines.

This study has shown that the Day 2 and Day 8 UK testing programme is successful in identifying traveller-related introduction of SARS-CoV-2 into the United Kingdom.  

We have detected a high prevalence 1.9% of cross-border transmission of SARS-CoV-2 involving a broad range of variants including variants of concern.  

The unexpectedly high prevalence of COVID-19 infection may reflect the lower sensitivity requirements for pre-embarkation testing, which includes lateral flow, having lower sensitivity when compared to PCR testing.  

The B.1.1.7 Kent variant was the most commonly detected among UK arrivals 80.6%, mirroring its high prevalence within the UK population.  

Due to its increased transmissibility, the B.1.1.7 Kent variant has spread from less than 0.1% in November 2020 to more than 95% of new SARS-CoV-2 infections in England in February 2021 and has now been detected globally in over 130 countries. 

The high diversity of 49 different SARS-CoV-2 variants we have found in international travellers including variants of concern and reflects of rapid biological evolution of SARS-CoV-2 leading to increased replication efficiency and transmissibility and potential for immune evasion.

This data shows that the vaccination programme has had a major impact in protecting individuals against infection, dropping from a prevalence of 2.4% in the unvaccinated population to 1.3% in the vaccinated population.  

Notably, vaccine effectiveness varies between first and second doses and age demographics.  

Analysis of vaccinated vs non-vaccinated groups with at least one dose shows overall effectiveness of 46%.  

However, this was significantly age-related, ranging from 67% in those less than 35 age group to 27% in those above 65 group.  

Analysis of the effect of the second dose revealed that vaccine effectiveness was significantly higher following second dose.  

Overall, one dose had a vaccine effectiveness of 32% compared with 78% in those who received their second dose of the vaccine.  

In the greater than 60 age group, the effectiveness for a single dose was 5% compared to 37% in less than 60 group.  

Notably, the effect of the second dose was significantly higher in both age groups with 60% effectiveness in those over 60 compared to 83% in those less than 60.  

This study has shown that the testing sequencing for international arrivals is playing a critical role in controlling the import of high-risk variants into the United Kingdom.

Monitoring cross-border transmission will become an increasing priority for many countries as vaccine programmes reach an advanced stage of roll out. 

However, the great public health benefits of vaccination programmes are at risk from the increasing numbers of variants of concern emerging as a result of rapidly increasing rates of global infections.  

The number of new cases each week has nearly doubled over the past two months.

Continuing globally high transmission rates of SARS-CoV-2 means the potential for the emergence of further variants with greater immunity-evading properties is high. 

Test and sequencing for international travellers combined with self-quarantine measures will therefore continue to play a public critical role in the public health measures required by countries to limit cross-border transmission of SARS-CoV-2 high risk variants and monitor potential future targets for vaccine. 

LGP, the leading London Doctors’ Clinic provides a full COVID-19 testing service

This includes:

Dr Paul Ettlinger

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