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## Let’s Clear This Up: What Does 95% Covid-19 Vaccine Efficacy Actually Mean?

In popular press and social media, there’s been a misunderstanding of what is actually meant when scientists say that the Pfizer and Moderna Covid-19 vaccines have an efficacy of 94-95%. It does not mean that 95% of vaccinated people are protected from infection โ these vaccines are better than that. Dr. Piero Olliaro explains in a letter to The Lancet:

The mRNA-based Pfizer and Moderna vaccines were shown to have 94-95% efficacy in preventing symptomatic COVID-19, calculated as 100 x (1 minus the attack rate with vaccine divided by the attack rate with placebo). It means that in a population such as the one enrolled in the trials, with a cumulated COVID-19 attack rate over a period of 3 months of about 1% without a vaccine, we would expect roughly 0.05% of vaccinated people would get diseased.

Another way to put it: you’re 20 times less likely to get Covid-19 with a vaccine than without. (And again, data indicates these are safe vaccines.) Olliaro explains with some simple math:

If we vaccinated a population of 100,000 and protected 95% of them, that would leave 5000 individuals diseased over 3 months, which is almost the current overall COVID-19 case rate in the UK. Rather, a 95% vaccine efficacy means that instead of 1000 COVID-19 cases in a population of 100,000 without vaccine (from the placebo arm of the abovementioned trials, approximately 1% would be ill with COVID-19 and 99% would not) we would expect 50 cases (99.95% of the population is disease-free, at least for 3 months).

And of course if you vaccinate widely, it becomes a compounding situation because the virus just runs out of people to infect.

## The Covid-19 Vaccines Are Amazing. Let’s Quickly Get Them into People’s Arms.

You probably read something yesterday, maybe just a headline, about Moderna’s Covid-19 vaccine being “six times less effective” against the B.1.351 coronavirus variant first identified in South Africa. This is, to put it plainly, a bullshit take on what is actually excellent news. This is the important bit, via Stat:

Both the Moderna vaccine and the immunization from Pfizer-BioNTech produce such powerful levels of immune protection โ generating higher levels of antibodies on average than people who recover from a Covid-19 infection have โ that they should be able to withstand some drop in their potency without really losing their ability to guard people from getting sick.

“There is a very slight, modest diminution in the efficacy of a vaccine against it, but there’s enough cushion with the vaccines that we have that we still consider them to be effective,” Anthony Fauci, the top U.S. infectious diseases official, said Monday on the “Today” show.

Let’s hear that again: “Both the Moderna vaccine and the immunization from Pfizer-BioNTech produce such powerful levels of immune protection…” These vaccines are so good, so potent, that even this sixfold drop in one measure of the vaccines’ ability to neutralize this one SARS-CoV-2 variant isn’t even enough to significantly reduce their overall protective power.1 That’s the important news here, that’s the very good news, that’s what you should be taking away from this. We have miraculously developed a near-perfect medicine for a plague that has significantly disrupted all human life on Earth and we’re flipping out over some technical details that the experts assure us don’t mean much in terms of overall effectiveness?! No thank you. Not today.

In a Twitter thread, Zeynep Tufekci is tearing her hair out because of the media’s misunderstanding and sensationalization of the “sixfold drop”.

I know people are tired but needless anxiety isn’t helping us. Let’s focus on getting through these months โ better masks if indoors with others, more strict attention to our precautions โ and the real problem: making more of these amazing vaccines quickly & getting them out there!

I get it, we want to understand but not how it works. Stop worrying about Nab titers. That does NOT mean the vaccine is six times less effective. People whose job it is to worry about it are on it & we just got confirmation: it works against the variants.

Plea to media: this isn’t a good headline. It makes people think the vaccine is six times less effective against the new variants (FALSE!) when the news today is *excellent*: The vaccine continues to work well against the new variants. That’s the headline.

For a much more technical take on the efficacy of the vaccines against variants, see virologist Florian Krammer’s long thread. His conclusion:

mRNA vaccines induce very high neutralizing antibodies after the second shot (consistently in the upper 25-30% of what we see with convalescent sera). If that activity is reduced by 10-fold, it is still decent neutralizing activity that will very likely protect. Furthermore, we know that the mRNA vaccines are already protective after the first shot when neutralizing antibody titers are low or undetectable in most individuals.

There is a concern here and it’s that B.1.351 or B.1.1.7 might mutate into variants that are significantly resistant against the vaccines’ good effects. Krammer again:

First, we need to do what every good scientist is praying for a year now: We need to cut down on virus circulation. The more the virus replicates, the more infections there are the higher are the chances for new variants to arise. Also, we need to try and contain B.1.351 and B.1.1.248/P.1 as much as possible.

That’s why, aside from preventing hundreds of thousands of deaths in the next several months, getting these vaccines into people’s arms is so important: the less the virus spreads, the less opportunity it will have to mutate into something even more dangerous. The US vaccination effort is slowly ramping up โ we’re at an average of 1.3 million doses per day right now and the trend is heading in the right direction. We can get this done!

1. Obviously, this could change! But the situation right now w/r/t variants is very good.โฉ

## Preliminary Results: Moderna Covid-19 Vaccine Is 94.5% Effective

Last Monday the world got some good news: an early review of the data showed that Pfizer’s Covid-19 vaccine was “more than 90% effective” in preventing the disease. The results pointed to other vaccines also being highly effective against the virus and this morning comes this news: Early Data Show Moderna’s Coronavirus Vaccine Is 94.5% Effective.

The drugmaker Moderna announced on Monday that its coronavirus vaccine was 94.5 percent effective, based on an early look at the results from its large, continuing study.

Researchers said the results were better than they had dared to imagine. But the vaccine will not be widely available for months, probably not until spring.

Despite the delivery timeline, this is such good news.

The companies’ products open the door to an entirely new way of creating vaccines โ and creating them fast. Both use a synthetic version of coronavirus genetic material, called messenger RNA or mRNA, to program a person’s cells to churn out many copies of a fragment of the virus. That fragment sets off alarms in the immune system and stimulates it to attack, should the real virus try to invade. Although a number of vaccines using this technology are in development for other infections and cancers, none have yet been approved or marketed.

“The fact that two different vaccines made by two different companies with two different kinds of structures, in a new messenger RNA concept, both worked so effectively confirms the concept once and for all that this is a viable strategy not only for Covid but for future infectious disease threats,” said Dr. Barry R. Bloom, a professor of public health at Harvard.

Natalie E. Dean, a biostatistician at the University of Florida, said an important finding was that the vaccine appeared to prevent severe disease. Pfizer did not release information about disease severity when reporting its results.

Researchers say the positive results from Pfizer and Moderna bode well for other vaccines, because all of the candidates being tested aim at the same target - the so-called spike protein on the coronavirus that it uses to invade human cells.

It’s only a few more months โ please please do what you can to stay safe and keep others safe (especially medical workers) until these vaccines can be rolled out.