The US Food and Drug Administration has been considering whether to approve a Covid-19 vaccine produced by Novavax, an American biotechnology company.
This vaccine was given approval by the European Medicines Agency in December and has already been widely used across Europe.
It was also approved by the UK’s Medicines and Healthcare products Regulatory Agency, in February.
Here we consider how the Novavax vaccine works, how it compares to other vaccines and how significant it might be in the fight against Covid-19.
How does the Novavax vaccine work?
The Novavax vaccine is a recombinant protein-based vaccine, which means that it involves genetic engineering technology and uses cells to produce the coronavirus spike protein.
In the case of the Novavax shot, an insect virus called a baculovirus is genetically engineered with a gene for the spike protein.
This baculovirus, Novavax explains in an online briefing document, are used to infect a type of moth cells called Sf9 cells.
The baculovirus multiplies inside the moth cells and the gene for the spike protein produces a type of genetic material called mRNA.
This mRNA causes the moth cells to produce large amounts of the coronavirus spike protein.
The proteins are purified and arranged around a tiny nanoparticle, which the company says helps “your immune system recognise the target spike”.
Novavax mixes these with an adjuvant, a substance that stimulates the immune system, which in this case comes from tree bark.
Once a person is injected with the vaccine, the immune system reacts against the spike proteins, resulting in a response that is protective in the event that the person is infected with the coronavirus.
How does Novavax compare to other vaccines?
The longest-established form of vaccine consists of the virus in a weakened form that is usually unable to cause disease.
In rare instances, particularly in people with compromised immune systems, such vaccines have led to illness.
Valneva, a French company, has developed a Covid-19 vaccine based on the inactivated coronavirus, although this has faced regulatory hurdles.
A later development was to use “dead” forms of the pathogen. While the risk of causing disease is eliminated, some such vaccines have not stimulated enough of an immune response.
The virus has to be grown in culture, which is easier for some than others, said Ian Jones, professor of virology at the University of Reading, so this can act as a technical hurdle for production.
A third type of vaccine involves genetic engineering and results in the production of proteins from the pathogen. They include the Novavax shot and “are quite widespread”, Prof Jones said.
“There are some for influenza. The vaccine for shingles, that’s a single recombinant protein. They have a very good safety record,” he said.
The technology behind recombinant protein vaccines is longer established than that used in the mRNA Covid-19 injections (such as Pfizer-BioNTech and Moderna), and the viral vector vaccines (such as Oxford-AstraZeneca and Janssen or Johnson & Johnson shots).
While mRNA and viral vector vaccines use newer technology, these vaccines have been extensively tested and found to be safe, with only rare serious side effects.
Do we need another Covid-19 vaccine?
With billions of doses of different types of Covid-19 vaccine already administered around the world, it raises the question of whether we need any more vaccines.
“The commonsense thing is that the more options available, the better. I know the vaccine has proved safe and effective in Europe,” said David Taylor, professor emeritus of pharmaceutical and public health policy at University College London.
Prof Taylor said that, theoretically, being able to identify very specific proteins that produce an immune response, as is the approach with recombinant protein vaccines, was the ideal approach, although in practice that was not always the case.
Prof Jones said the coronavirus was still circulating and continued to cause serious illness in some people, so vaccines were still needed.
He said the Novavax vaccine might be more appealing to people who had concerns about receiving some of the existing Covid-19 injections, for example because they were based on newer technology.
“They may feel reassured that this version is using a technology that has been established for many other things," Prof Jones said. "There will be a class of individuals who feel happier with this form of vaccine."