Autumn 2023 sees another round of COVID-19 booster jabs across the UK for vulnerable people.
This new round of booster jabs will predominantly employ the mRNA vaccine technology that found widespread use in 2020-21 during the COVID-19 pandemic. However, the actual mRNA vaccines being used this autumn have been adapted and updated to provide optimal protection as the SARS-CoV-2 virus continues to evolve and mutate.
The original first-generation mRNA vaccines elasomeran (Comirnaty®) and tozinameran (Spikevax ®) produced by BioNTech/Pfizer and Moderna respectively were first administered to the public in the UK in Dec 2020 and Jan 2021 respectively. Both vaccines offered significant levels of protection against the original SARS-CoV-2 virus that was first identified in China in Dec 2019 and was in wide circulation in the UK population at the time.
One advantage of mRNA over other vaccine technologies is that synthetic mRNA molecules can be synthesised quickly and easily. Not only did this allow mRNA vaccines to be quickly developed following the initial characterisation of the SARS-CoV-2 virus, but it also allows the vaccines to be updated to target new SARS-CoV-2 variants. mRNA vaccine technology is also amenable to multi-valency as multiple mRNA coding sequences for different antigens can be synthesised and delivered simultaneously. For example, the mRNA cancer vaccine mRNA-4157/V940 (Moderna) targets up to 34 different antigens. A multivalent COVID-19 vaccine might target multiple different variants of the SARS-CoV-2 virus.
Given these advantages, it is not surprising that when second-generation mRNA vaccines were developed for the 2022 round of booster jabs, bivalent vaccines for used. In addition to mRNA coding sequence for the Spike antigen from the original SARS-CoV-2 virus, these second-generation vaccines also contained mRNA coding sequences for the Spike antigen from the Omicron variant BA.1 or the Omicron variant BA.4-5. These Omicron variants were in broad circulation at the time. These bivalent second-generation mRNA vaccines again provided to be highly effective.
Despite the relative ease of production of multi-valent mRNA vaccines, this Autumn sees a move away from this approach. The third-generation mRNA vaccines that will be used for the 2023 round of boosters jabs will be mono-valent vaccines.
This move is based on findings that monovalent vaccines offer better immune responses than bivalent vaccines against the XBB sub-lineages of the Omicron variant that are currently the globally dominant forms of SARS-CoV-2. The original SARS-CoV-2 virus is no longer in circulation in the population and the mRNA coding sequences for its Spike antigen do not elicit host antibodies that are reactive against the more recent Omicron variants. There is therefore no benefit in including it in a COVID-19 vaccine for 2023. Multi-valent vaccines have also been found to generate weaker immune responses against new SARS-CoV-2 variants than mono-valent antibodies.
Whilst multi-valent mRNA vaccines are likely to remain important in other fields, notably cancer, the switch to mono-valent vaccines for the Autumn 2023 round of COVID-19 booster jabs is a sign that multivalent vaccines are unlikely to be in the front line of our defence against COVID-19 in the future. However, the ability to adapt and update mRNA vaccines is likely to remain highly valuable in targeting the particular SARS-CoV-2 variants that are circulating within the population.
