The results for the phase III trial of the Novavax Covid vaccine are now out, and the news seems very good – an overall efficacy of about 90% in the UK trial, with complete protection against severe disease and death. The prospects now look very promising for regulatory approval. What’s striking about this is that we now have a third, completely different class of vaccine that has demonstrated efficacy against COVID-19. We have the mRNA vaccines from BioNTech/Pfizer and Moderna, the viral vector vaccine from Oxford/AstraZeneca, and now Novavax, which is described as “recombinant nanoparticle technology”. As I’ve discussed before (in Nanomedicine comes of age with mRNA vaccines), the Moderna and BioNTech/Pfizer vaccines both crucially depend on a rather sophisticated nanoparticle system that wraps up the mRNA and delivers it to the cell. The Novavax vaccine depends on nanoparticles, too, but it turns out that these are rather different in their character and function to those in the mRNA vaccines – and, to be fair, are somewhat less precisely engineered. So what are these “recombinant nanoparticles”?
All three of these vaccine classes – mRNA, viral vector and Novavax – are based around raising an immune response to a particular protein on the surface of the coronovirus – the so-called “spike” protein, which binds to receptors on the surface of target cells at the start of the process through which the virus makes its entrance. The mRNA vaccines and the viral vector vaccines both hijack the mechanisms of our own cells to get them to produce analogues of these spike proteins in situ. The Novavax vaccine is less subtle – the protein itself is used as the vaccine active ingredient. It’s synthesised in bioreactors by using a genetically engineered insect virus, which is used to infect a culture of cells from a moth caterpillar. The infected cells are harvested and the spike proteins collected and formulated. It’s this stage that, in the UK, will be carried out in the Teeside factory of the contract manufacturer Fujifilm Diosynth Biotechnologies.
The protein used in the vaccine is a slightly tweaked version of the molecule in the coronavirus. The optimal alteration was found by Novavax’s team, led by scientist Nita Patel, who quickly tried out 20 different versions before hitting on the variety that is most stable and immunologically active. The protein has two complications compared to the simplest molecules studied by structural biologists – it’s a glycoprotein, which means that it has short polysaccharide chains attached at various points along the molecule, and it’s a membrane protein (this means that it’s structure has to be determined by cryo-transmission electron microscopy, rather than X-ray diffraction). It has a hydrophobic stalk, which sticks into the middle of the lipid membrane which coats the coronavirus, and an active part, the “spike”, attached to this, sticking out into the water around the virus. For the protein to work as a vaccine, it has to have exactly the same shape as the spike protein has when it’s on the surface of the virus. Moreover, that shape changes when the virus approaches the cell it is going to infect – so for best results the protein in the vaccine needs to look like the spike protein at the moment when it’s armed and ready to invade the cell.
This is where the nanoparticle comes in. The spike protein is formulated with a soap-like molecule called Polysorbate 80 (aka Tween 80). This consists of a hydrocarbon tail – essentially the tail group of oleic acid – attached to a sugar like molecule – sorbitan – to which are attached short chains of ethylene oxide. The whole thing is what’s known as a non-ionic surfactant. It’s like soap, in that it has a hydrophobic tail group and a hydrophilic head group. But unlike soap or comment synthetic detergents, the head group is, although water soluble, uncharged. The net result is that in water Polysorbates-80 self-assembles into nanoscale droplets – micelles – in which the hydrophobic tails are buried in the core and the hydrophilic head groups cover the surface, interacting with the surrounding water. The shape and size of the micelles is set by the length of the tail group and the area of the head group, so for these molecules the optimum shape is a sphere, probably a few tens of nanometers in diameter.
As far as the spike proteins are concerned, these somewhat squishy nanoparticles look a bit like the membrane of the virus, in that they have an oily core that the stalks can be buried in. When the protein, having been harvested from the insect cells and purified, is mixed up with a polysorbate-80 solution, they end up stuck into the sphere like a bunch of whole cloves stuck into a mandarin orange. Typically each nanoparticle will have about 14 spikes. It has to be said that, in contrast to the nanoparticles carrying the mRNA in the BioNTech and Moderna vaccines, neither the component materials nor the process for making the nanoparticles is particularly specialised. Polysorbate-80 is a very widely used, and very cheap, chemical, extensively used as an emulsifier in convenience food and an ingredient in cosmetics, as well as in many other pharmaceutical formulations, and the formation of the nanoparticles probably happens spontaneously on mixing (though I’m sure there are some proprietary twists and tricks to get it to work properly, there usually are).
But the recombinant protein nanoparticles aren’t the only nanoparticles of importance in the Novavax vaccine. It turns out that simply injecting a protein as an antigen doesn’t usually provoke a strong enough immune response to work as a good vaccine. In addition, one needs to use one of the slightly mysterious substances called “adjuvants” – chemicals that, through mechanisms that are probably still not completely understood, prime the body’s immune system and provoke it to make a stronger response. The Novavax vaccine uses as an adjuvant another nanoparticle – a complex of cholesterol and phospholipid (major components of our own cell membranes, widely available commercially) together with molecules called saponins, which are derived from the Chilean soap-bark tree.
Similar systems have been used in other vaccines, both for animal diseases (notably foot and mouth) and human. The Novavax adjuvant technology was developed by a Swedish company, Isconova AB, which was bought by Novavax in 2013, and consists of two separate fractions of Quillaja saponins, separately formulated into 40 nm nanoparticles and mixed together. The Chilean soap-bark tree is commercially cultivated – the raw extract is used, for example, in the making of the traditional US soft drink, root beer – but production will need to be stepped up (and possibly redirected from fizzy drinks to vaccines) if these vaccines turn out to be as successful as it now seems they might.
Sources: This feature article on Novavax in Science is very informative, but I believe the cartoon depicting the nanoparticle isn’t likely to be accurate, depicting it as cylindrical when it is much more likely to be spherical, and based on double tailed lipids rather than the single tailed anionic surfactant that is in fact used in the formulation. This is the most detailed scientific article from the Novavax scientists describing the vaccine and its characterisation. The detailed nanostructure of the vaccine protein in its formulation is described in this recent Science article. The “Matrix-M” adjuvant is described here, while the story of the Chilean soap-bark tree and its products is described in this very nice article in The Atlantic Magazine.