With the already staggering coronavirus case and death toll expected to climb further in the US following Thanksgiving, there’s at least more good news on the vaccine front. The biotech firm Moderna released the final results of its 30,000-person vaccine trial in a press release Monday, reporting a 94.1 percent rate of efficacy. The finding squares with the 94.5 percent efficacy rate the company reported two weeks ago, based on its first interim analysis of trial data.

Of the 196 Covid-19 cases in the trial, 185 were in the placebo group and only 11 in the vaccine group, Moderna reported.

Even more important, the vaccine — called mRNA-1273 — appears to protect against severe disease, not just asymptomatic or mild cases. Of the 30 severe Covid-19 cases among trial participants, all occurred in the placebo group. If the finding is real, it would likely mean averted deaths and hospitalizations when millions of people are immunized.

“You’ve got 100 percent protection against severe disease,” Paul Offit, an infectious disease and vaccine researcher at Children’s Hospital of Philadelphia, told Vox. “That’s remarkable.”

“If these numbers are right, it’s more than we’d need for the vaccine to be a major control measure for this outbreak,” said Eric Rubin, an infectious disease specialist and the editor-in-chief of the New England Journal of Medicine.

Moderna’s CEO, Stéphane Bancel, said in the press release that the company plans to request an Emergency Use Authorization from the Food and Drug Administration, which would allow the vaccine to be used in limited cases for people facing some of the highest risk of exposure to Covid-19, like health care workers. “We believe that our vaccine will provide a new and powerful tool that may change the course of this pandemic and help prevent severe disease, hospitalizations and death,” he added.

Given that the Pfizer/BioNTech and AstraZeneca/Oxford coronavirus vaccine research groups have also put out promising findings recently, this latest announcement of final data from the Moderna trial reaffirms that the world will likely have several highly effective vaccines for Covid-19 — and the end of the pandemic may be on the horizon. High efficacy also means that fewer people would need to be vaccinated in order to achieve herd immunity, the threshold at which the virus can no longer spread easily from person to person.

But, as always, there are caveats. In this case, the vaccine requires two doses, there are some side effects, and we don’t yet have details about how the vaccine worked in high-risk groups. And while demonstrating efficacy is important, the road to getting millions of people vaccinated will be fraught with logistical challenges. A lot of difficult work on a Covid-19 vaccine still lies ahead.

How Moderna showed that its Covid-19 vaccine works

Moderna’s announcement of 94.1 percent efficacy is based on a phase 3 clinical trial. In particular, the results are from the COVE study, conducted in collaboration with the US government’s National Institute of Allergy and Infectious Diseases (NIAID) and the Biomedical Advanced Research and Development Authority (BARDA).

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Phase 3 is where the vaccine is tested against the virus spreading in the real world. Since experimenters can’t deliberately infect people, they have to wait and see who gets sick with Covid-19 in their volunteer pool, comparing the group that received the actual vaccine to the group that received the placebo. Moderna’s vaccine is administered as two doses.

To speed up the process, researchers recruit thousands of volunteers so that the rate of accumulating infections goes up. But it only takes a handful of infections to demonstrate that the vaccine works.

If a vaccine doesn’t work, and half the people in the trial get the vaccine and the other half get the placebo, we’d expect coronavirus cases to be evenly split in the two groups, Natalie Dean, an assistant professor of biostatistics at the University of Florida, told Vox. But when a vaccine is effective, we get results like the ones Moderna is reporting.

According to the biotech firm, experimenters detected 11 cases in people who received two doses of the vaccine compared to 185 in the placebo group. This shows that the virus was spreading among volunteers in the clinical trial but was drastically lower among those who received Moderna’s vaccine. “When we think about the level of evidence, this is a strong result,” Dean said.

There are some caveats to Moderna’s Covid-19 vaccine results

Moderna’s latest results were announced in a press release, and came directly from the company. While there have been several peer-reviewed interim studies about its vaccine, Monday’s announcement of final results didn’t arrive with any published data — just like the November 16 announcement of interim results.

That doesn’t mean the findings are wrong, but they lack critical details and nuances we need to interpret them — like how well the vaccine works if people get only one dose (not an unlikely scenario in the real world) and how effective it is in high-risk groups.

We don’t know anything on the former, and on the latter, Moderna has only reported that “efficacy was consistent across age, race and ethnicity, and gender demographics.” While the company provided numbers for how many trial participants were over 65 and from various ethnic communities, the company didn’t say how the vaccine performed in each of these subgroups. That information is critical, since these are the people who’ve been hardest hit by the virus.

We also don’t yet know how long people who got the vaccine remain protected from the virus, Dean pointed out. There are also the side effects to consider. Moderna has reported no serious safety issues to date and said that most problems tended to be mild to moderate — but up to 10 percent of participants experienced severe side effects, according to an earlier press release. These included fatigue (9.7 percent), muscle pain (8.9 percent), joint pain (5.2 percent), headache (4.5 percent), other pain (4.1 percent), and redness at the injection site (2 percent).

Since the vaccines will ultimately have to be distributed to millions, if not billions, of people, it’s important to pay attention to side effects. Rare complications will be more likely to show up once lots of people get the shot. And clinical trials of other Covid-19 vaccine candidates — like the Johnson & Johnson vaccine or the University of Oxford and AstraZeneca vaccine — have already been paused due to complications among recipients.

What happens next for a Covid-19 vaccine

If the Moderna vaccine receives emergency approval in the US, distribution could begin in December. Bancel, the Moderna CEO, told Science that the company plans to charge $32 to $37 per dose of the vaccine in developed countries. But while the company says it will have 20 million doses ready by the end of 2020 for the US market, distributing the vaccine will be challenging.

Moderna’s vaccine requires long-term storage at minus 20 degrees Celsius (minus 4 degrees Fahrenheit) and is stable for 30 days between 2 and 8 degrees Celsius (36 degrees to 46 degrees Fahrenheit). That’s well within the temperature range of conventional refrigerators and warmer than the temperature requirements of the Pfizer/BioNTech shot, but it may still pose a logistical hurdle in some lower-resource settings, like rural hospitals, that lack certain kinds of cold storage facilities.

Moderna’s is also a two-dose vaccine, which means every recipient needs to come back for a second injection to get that high rate of efficacy. We know from other multi-dose vaccines that not everybody will return for that second shot — and the efficacy profile may look different. “When you do an experiment, it’s done under best conditions,” Offit said. “When things roll out in the real world, in real-world conditions, there’s a fraying.”

It’s also important to remember that an effective vaccine is not enough to end the pandemic quickly. Measures like social distancing, practicing good hygiene, and wearing face masks will remain essential to control the spread of Covid-19 until a vaccine is widely available. Public acceptance may also be an issue, and health officials will have to overcome a rising wave of vaccine hesitancy.

Vaccine research also doesn’t end once a vaccine is rolled out. Public health officials, doctors, and the biotech companies will still have to track complications across millions of people and pay attention to how quickly immunity wanes.

So far, we have two months of safety data after the second dose, and while that’s not long-term, it should build confidence, Offit said. “Serious side effects usually shot up within six weeks of the second dose. 260,000 people have died this year in the US [of Covid-19]. It would be great if we could do a three- to four-year study and look at length of efficacy and the duration of efficacy. But the question isn’t when do you know everything here — it’s when do you know enough.”

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