A new vaccine for Covid-19 that is entering clinical trials in Brazil, Mexico, Thailand and Vietnam could change how the world fights the pandemic. The vaccine, called NDV-HXP-S, is the first in clinical trials to use a new molecular design that is widely expected to create more potent antibodies than the current generation of vaccines. And the new vaccine could be far easier to make. Existing vaccines from companies like Pfizer and Johnson & Johnson must be produced in specialised factories using hard-to-acquire ingredients. In contrast, the new vaccine can be mass-produced in chicken eggs — the same eggs that produce billions of influenza vaccines every year in factories around the world.
If NDV-HXP-S proves safe and effective, flu vaccine manufacturers could potentially produce well over a billion doses of it a year. Low- and middle-income countries currently struggling to obtain vaccines from wealthier countries may be able to make NDV-HXP-S for themselves or acquire it at low cost from neighbours. “That’s staggering — it would be a game-changer,” said Andrea Taylor, assistant director of the Duke Global Health Innovation Center. First, however, clinical trials must establish that NDV-HXP-S actually works in people. The first phase of clinical trials will conclude in July, and the final phase will take several months more. But experiments with vaccinated animals have raised hopes for the vaccine’s prospects. “It’s a home run for protection,” said Dr. Bruce Innis of the PATH Center for Vaccine Innovation and Access, which has coordinated the development of NDV-HXP-S. “I think it’s a world-class vaccine.”
Vaccines work by acquainting the immune system with a virus well enough to prompt a defense against it. Some vaccines contain entire viruses that have been killed; others contain just a single protein from the virus. Still others contain genetic instructions that our cells can use to make the viral protein.
Once exposed to a virus, or part of it, the immune system can learn to make antibodies that attack it. Immune cells can also learn to recognise infected cells and destroy them.
In the case of the coronavirus, the best target for the immune system is the protein that covers its surface like a crown. The protein, known as spike, latches onto cells and then allows the virus to fuse to them. But simply injecting coronavirus spike proteins into people is not the best way to vaccinate them. That’s because spike proteins sometimes assume the wrong shape, and prompt the immune system to make the wrong antibodies. This insight emerged long before the Covid-19 pandemic. In 2015, another coronavirus appeared, causing a deadly form of pneumonia called MERS. Jason McLellan, a structural biologist then at the Geisel School of Medicine at Dartmouth, and his colleagues set out to make a vaccine against it.
They wanted to use the spike protein as a target. But they had to reckon with the fact that the spike protein is a shape-shifter. As the protein prepares to fuse to a cell, it contorts from a tulip-like shape into something more akin to a javelin. Scientists call these two shapes the prefusion and postfusion forms of the spike. Antibodies against the prefusion shape work powerfully against the coronavirus, but postfusion antibodies don’t stop it.
Dr. McLellan and his colleagues used standard techniques to make a MERS vaccine but ended up with a lot of postfusion spikes, useless for their purposes. Then they discovered a way to keep the protein locked in a tulip-like prefusion shape. All they had to do was change two of more than 1,000 building blocks in the protein into a compound called proline.
The resulting spike — called 2P, for the two new proline molecules it contained — was far more likely to assume the desired tulip shape. The researchers injected the 2P spikes into mice and found that the animals could easily fight off infections of the MERS coronavirus.
The team filed a patent for its modified spike, but the world took little notice of the invention. MERS, although deadly, is not very contagious and proved to be a relatively minor threat; fewer than 1,000 people have died of MERS since it first emerged in humans.
But in late 2019 a new coronavirus, SARS-CoV-2, emerged and began ravaging the world. Dr. McLellan and his colleagues swung into action, designing a 2P spike unique to SARS-CoV-2. In a matter of days, Moderna used that information to design a vaccine for Covid-19; it contained a genetic molecule called RNA with the instructions for making the 2P spike.
Other companies soon followed suit, adopting 2P spikes for their own vaccine designs and starting clinical trials. All three of the vaccines that have been authorised so far in the United States — from Johnson & Johnson, Moderna and Pfizer-BioNTech — use the 2P spike.
Other vaccine makers are using it as well. Novavax has had strong results with the 2P spike in clinical trials and is expected to apply to the Food and Drug Administration for emergency use authorisation in the next few weeks. Sanofi is also testing a 2P spike vaccine and expects to finish clinical trials later this year. Dr. McLellan’s ability to find lifesaving clues in the structure of proteins has earned him deep admiration in the vaccine world. “This guy is a genius,” said Harry Kleanthous, a senior program officer at the Bill & Melinda Gates Foundation. “He should be proud of this huge thing he’s done for humanity.” But once Dr. McLellan and his colleagues handed off the 2P spike to vaccine makers, he turned back to the protein for a closer look. If swapping just two prolines improved a vaccine, surely additional tweaks could improve it even more.
“It made sense to try to have a better vaccine,” said Dr. McLellan, who is now an associate professor at the University of Texas at Austin. In March, he joined forces with two fellow University of Texas biologists, Ilya Finkelstein and Jennifer Maynard. Their three labs created 100 new spikes, each with an altered building block. With funding from the Gates Foundation, they tested each one and then combined the promising changes in new spikes. Eventually, they created a single protein that met their aspirations. The winner contained the two prolines in the 2P spike, plus four additional prolines found elsewhere in the protein. Dr. McLellan called the new spike HexaPro, in honor of its total of six prolines.
The structure of HexaPro was even more stable than 2P, the team found. It was also resilient, better able to withstand heat and damaging chemicals. Dr. McLellan hoped that its rugged design would make it potent in a vaccine. Dr. McLellan also hoped that HexaPro-based vaccines would reach more of the world — especially low- and middle-income countries, which so far have received only a fraction of the total distribution of first-wave vaccines. “The share of the vaccines they’ve received so far is terrible,” Dr. McLellan said. To that end, the University of Texas set up a licensing arrangement for HexaPro that allows companies and labs in 80 low- and middle-income countries to use the protein in their vaccines without paying royalties.
Carl Zimmer writes the “Matter” column for NYT©2021