The End of the Pandemic Is Now in Sight

A year of scientific uncertainty is over. Two vaccines look like they will work, and more should follow.

By Sarah Zhang

For all that scientists have done to tame the biological world, there are still things that lie outside the realm of human knowledge. The coronavirus was one such alarming reminder, when it emerged with murky origins in late 2019 and found naive, unwitting hosts in the human body. Even as science began to unravel many of the virus’s mysteries — how it spreads, how it tricks its way into cells, how it kills — a fundamental unknown about vaccines hung over the pandemic and our collective human fate: Vaccines can stop many, but not all, viruses. Could they stop this one?

But it is no accident or surprise that Moderna and Pfizer are first out of the gate. They both bet on a new and hitherto unproven idea of using mRNA, which has the long-promised advantage of speed. This idea has now survived a trial by pandemic and emerged likely triumphant. If mRNA vaccines help end the pandemic and restore normal life, they may also usher in a new era for vaccine development.

mRNA vaccines offer a clever shortcut. We humans don’t need to intellectually work out how to make viruses; our bodies are already very, very good at incubating them. When the coronavirus infects us, it hijacks our cellular machinery, turning our cells into miniature factories that churn out infectious viruses. The mRNA vaccine makes this vulnerability into a strength. What if we can trick our own cells into making just one individually harmless, though very recognizable, viral protein? The coronavirus’s spike protein fits this description, and the instructions for making it can be encoded into genetic material called mRNA.

This overall process mimics the steps of infection better than some traditional vaccines, which suggests that mRNA vaccines may provoke a better immune response for certain diseases. When you inject vaccines made of inactivated viruses or viral pieces, they can’t get inside the cell, and the cell can’t present those viral pieces to the immune system. Those vaccines can still elicit proteins called antibodies, which neutralize the virus, but they have a harder time stimulating T cells, which make up another important part of the immune response. (Weakened viruses used in vaccines can get inside cells, but risk causing an actual infection if something goes awry. mRNA vaccines cannot cause infection because they do not contain the whole virus.) Moreover, inactivated viruses or viral pieces tend to disappear from the body within a day, but mRNA vaccines can continue to produce spike protein for two weeks, says Drew Weissman, an immunologist at the University of Pennsylvania, whose mRNA vaccine research has been licensed by both BioNTech and Moderna. The longer the spike protein is around, the better for an immune response.

The spectacular efficacy of these vaccines, should the preliminary data hold, likely also has to do with the choice of spike protein as vaccine target. On one hand, scientists were prepared for the spike protein, thanks to research like Graham’s. On the other hand, the coronavirus’s spike protein offered an opening. Three separate components of the immune system — antibodies, helper cells, and killer T cells — all respond to the spike protein, which isn’t the case with most viruses.

In this, we were lucky. “It’s the three punches,” says Alessandro Sette. Working with Shane Crotty, his fellow immunologist at the La Jolla Institute, Sette found that COVID-19 patients whose immune systems can marshal all three responses against the spike protein tend to fare the best. The fact that most people can recover from COVID-19 was always encouraging news; it meant a vaccine simply needed to jump-start the immune system, which could then take on the virus itself. But no definitive piece of evidence existed that proved COVID-19 vaccines would be a slam dunk. “There’s nothing like a Phase 3 clinical trial,” Crotty says. “You don’t know what’s gonna happen with a vaccine until it happens, because the virus is complicated and the immune system is complicated.”

Experts anticipate that the ongoing trials will clarify still-unanswered questions about the COVID-19 vaccines. For example, Ruth Karron, the director of the Center for Immunization Research at Johns Hopkins University, asks, does the vaccine prevent only a patient’s symptoms? Or does it keep them from spreading the virus? How long will immunity last? How well does it protect the elderly, many of whom have a weaker response to the flu vaccine? So far, Pfizer has noted that its vaccine seems to protect the elderly just as well, which is good news because they are especially vulnerable to COVID-19.

The vaccine by itself cannot slow the dangerous trajectory of COVID-19 hospitalizations this fall or save the many people who may die by Christmas. But it can give us hope that the pandemic will end. Every infection we prevent now — through masking and social distancing — is an infection that can, eventually, be prevented forever through vaccines.

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