The UK's Medicines and Healthcare products Regulatory Agency (MHRA) has approved a bivalent COVID booster vaccine, making it the first country in the world to do so.

Developed by Moderna, this vaccine has been approved for use in adults, and is set to form part of the upcoming autumn booster campaign in the UK.

But what actually is a bivalent vaccine, and what impact might this booster have on the trajectory of the pandemic? Let's take a look.

Read more: COVID vaccines: our current shots could soon be updated to target new variants - an immunology expert explains

The COVID vaccines and boosters we currently have, or the first generation, are "monovalent" vaccines. This means they only target the original strain of SARS-CoV-2 (the virus that causes COVID-19).

A recent study suggested that first generation COVID vaccines prevented up to 20 million deaths around the world in their first year of use.

The emergence of the new variants of SARS-CoV-2, including omicron variants, has been very concerning. Omicron variants are better than earlier non-omicron variants at evading our immunity from prior infections and vaccines.

Although the vaccines continue to offer protection against deaths and hospital admissions, recent research has shown the initial course of COVID vaccination provides limited protection against symptomatic disease caused by the omicron variant.

So this second-generation bivalent or dual-variant vaccine targets both the ancestral strain of SARS-CoV-2 and the omicron variant BA.1. It contains 25 micrograms of original coronavirus vaccine and 25 micrograms of vaccine that specifically targets the omicron variant.

Likewise, "multivalent" vaccines can protect against even more than two strains of a microbe - though we don't have any multivalent shots for COVID yet.

Bivalent and multivalent vaccines aren't new in healthcare. For example, all influenza vaccines available in the UK and the US are quadrivalent, targeting four different strains of flu.

The Gardasil-9 vaccine targets nine strains of the human papillomavirus, a common sexually transmitted infection. Meanwhile, the pneumococcal polysaccharide vaccine, PPSV23, targets an impressive 23 different strains of bacteria that cause pneumococcal diseases, protecting against pneumonia and meningitis, among others.

Safety and efficacy

No serious safety concerns have been identified for this new bivalent vaccine. Any side effects observed during safety monitoring were broadly the same as those seen with the original Moderna booster dose. These are typically mild and get better on their own, such as fever, headache, fatigue or pain at the injection site.

The MHRA's approval of Moderna's new bivalent vaccine is based on data from a clinical trial involving more than 400 participants. The results showed that a booster of the bivalent vaccine triggers a strong immune response against both the original Wuhan strain and omicron BA.1.

Specifically, Moderna reported that in a combined phase 2 and 3 trial, a booster dose of the new bivalent vaccine increased neutralising antibody levels against omicron BA.1 roughly eight-fold above baseline levels. This was a superior neutralising antibody response when compared with the company's current monovalent booster.

Notably, omicron BA.1 was the first omicron subvariant, but BA.5 is now the dominant variant in the UK and globally. While this vaccine was designed to target BA.1, Moderna have stated the booster also elicited potent neutralising antibody responses against BA.4 and BA.5 compared with the company's current booster.

These results are promising, but we will need to watch closely for confirmation that they translate beyond clinical trials, and that the vaccine is effective against BA.5 and potentially any new variants in the real word.

In the meantime, Moderna has completed regulatory submissions for its bivalent vaccine in other countries including Canada, Australia and the European Union. Pending authorisation, the vaccine is likely to be available in the US starting in autumn too.

Other pharmaceutical companies including Pfizer and BioNTech are also developing and trialling bivalent boosters to target omicron.

Read more: COVID vaccines: why second boosters are being offered to vulnerable people in the UK - but not young and healthy people yet

Boosters are a vital weapon against COVID

When immunity from initial vaccine doses wanes, boosters are an important way to increase our immunity. And there's little doubt that introducing this new Moderna bivalent vaccine will provide substantial protection to many people against COVID, including the newer variants, as we enter the winter months.

At the same time, first-generation boosters are still highly valuable, alongside other precautions we can continue taking to prevent the spread of the virus and new variants. These might include wearing a mask in crowded places, staying away from others when ill, and maintaining good hand hygiene.

COVID remains a threat and we can't anticipate how the virus will evolve. We may well see the emergence of new variants, creating a need for multivalent vaccines in the future.

Author: Manal Mohammed - Senior Lecturer, Medical Microbiology, University of Westminster