Subunit vaccines use a small part of a pathogen to train our immune system to fight off future infection. They cannot cause disease but may need the addition of other chemicals for the vaccine to trigger a strong immune reaction.

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Vaccines equip the immune system to recognize future infections with pathogens. Some vaccines use the whole pathogen to do this, but others only use one part.

Subunit vaccines, such as the Novavax COVID-19 vaccine candidate, usually contain either a protein, a polysaccharide — a sugar molecule, or a combination of the two from a pathogen. They are safe because they cannot cause disease.

Because these vaccines contain only a small part of the pathogen, they are less likely to produce strong side effects. However, they are also less likely to create long-lasting immunity.

Therefore, many subunit vaccines contain adjuvants, chemicals that scientists add to the vaccine to elicit a robust immune response.

It is possible to create longer-lasting immunity by combining the small part of the pathogen with an adjuvant. Some subunit vaccines also require booster doses to ensure lasting protection from the disease.

Examples of subunit vaccines are the whooping cough and hepatitis B vaccines.

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The experimental COVID-19 vaccine that Novavax are developing is a recombinant subunit vaccine, meaning that its SARS-CoV-2 components are created in a laboratory rather than isolated directly from the virus.

The vaccine candidate, which bears the name NVX-CoV2373, is currently undergoing phase 3 clinical trials.

Novavax manufacture their vaccine candidate by growing large amounts of the SARS-CoV-2 spike protein in insect cells. The proteins then undergo purification from the insect cells, after which another process converts them into nanoparticles.

On their own, each spike protein would be too small to generate a robust immune response. But as a nanoparticle, this is more likely to occur. Novavax also include an adjuvant in their experimental vaccine to ensure the immune system reacts strongly to the injection.

The vaccine candidate cannot cause COVID-19 because it does not contain enough SARS-CoV-2 components to allow our bodies to assemble the entire virus.

According to a press release, the first interim analysis of a phase 3 clinical trial based in the United Kingdom showed that the Novavax vaccine candidate had an efficacy of 89.3%. The researchers also looked at the data in light of the emerging B.1.1.7 SARS-CoV-2 variant.

Against the previously dominant SARS-CoV-2 variant, the vaccine showed a 95.6% efficacy. For infections with the B.1.1.7 variant, the efficacy was 85.6%.

According to Reuters, Novavax expect the results of their large-scale phase 3 trial in the United States at the beginning of April. At this point, the company plan to apply to the Food and Drug Administration (FDA) for Emergency Use Authorization.

There are two other COVID-19 subunit vaccine candidates currently undergoing phase 3 trials. One is called RBD-Dimer, in co-development by Anhui Zhifei Longcom Biopharmaceutical and the Institute of Microbiology Chinese Academy of Sciences.

The other is known as EpiVacCorona, developed by scientists at the Russian Vector Institute in Siberia. Although this vaccine has authorization for use in Russia, there are no data available from large-scale clinical trials yet.

Researchers are using well-established technology to make these subunit vaccines, although each pharmaceutical company will have its proprietary method for making their particular vaccine.

One clear advantage is that subunit vaccines do not require sophisticated storage conditions.

The Novavax COVID-19 vaccine candidate can be stored at regular refrigerator temperatures, making it particularly suitable for rollout in parts of the world where ultracold storage proves challenging.

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