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  • [♪ INTRO]

  • This episode was filmed on January 26, 2021.

  • For up-to-date information on the COVID-19 pandemic,

  • check out our playlist linked in the description.

  • In some ways, designing and testing vaccines for COVID-19 was the easy part.

  • Now, we face the monumental task of getting those vaccines out to people.

  • And different nations are tackling that effort in different ways.

  • And adding to the difficulty is the fact that

  • many of the vaccines we have so far were tested using multiple doses.

  • For instance, the vaccine from BioNTech and Pfizer requires two shots, three weeks apart.

  • The government of the United Kingdom, however,

  • has made the calculation that one dose is better than none, and have devised a plan

  • to stretch limited vaccine supplies to reach as many individuals as possible.

  • To that end, they plan to delay any second doses of the BioNTech vaccine

  • by up to 12 weeks, not 3.

  • And, in rare cases, they may even mix and match,

  • like giving someone their first dose of the BioNTech vaccine,

  • and the second dose of a totally different one.

  • A few other countries are following suit, or thinking about it.

  • And the U.S. Centers for Disease Control has updated their guidelines

  • to allow for up to six weeks between shots for the BioNTech/Pfizer vaccine.

  • They're also allowing six weeks for another vaccine by the manufacturer Moderna,

  • which would normally be spaced only four weeks apart.

  • Sowill this work? The short answer is: this is a gamble.

  • And one that's not necessarily backed by the data we have so far.

  • Two doses is not unusual for a vaccine in general.

  • In fact, it's common to have one dose followed by a secondboosterdose of the same vaccine.

  • Vaccines teach your body to spot invaders, or parts of invaders,

  • by triggering your immune system, which creates antibodies that bind to foreign material.

  • This process creates a kind of memory in your immune system,

  • one that can fade over time after the first exposure.

  • But by giving our bodies a second exposure to the threat,

  • we essentially convince our immune system to take it seriously.

  • The repeat dose convinces our bodies to keep that memory around for a long time.

  • Many of our new COVID vaccines work according to this scheme.

  • The BioNTech and Moderna vaccines use two doses.

  • So does a third vaccine from Oxford and AstraZeneca.

  • This is what early testing found would provide the best immune response.

  • Later clinical trials built up from there,

  • and evidence from those trials is what convinced the various regulatory agencies

  • of the world that these vaccines were safe and effective enough for emergency use.

  • In those trials, BioNTech's doses were given three weeks apart.

  • Moderna's were given four weeks apart.

  • But, like we said, countries are looking at waiting as long as six or even 12 weeks between doses.

  • And it's hard to predict what effect delaying the second vaccine dose will have.

  • For one thing, protection isn't linear.

  • You aren't half-protected after getting half the shots.

  • We also don't know yet whether COVID vaccines completely stop infection,

  • or just keep people from getting sick.

  • And BioNTech/Pfizer and Moderna's clinical trials weren't designed to test

  • using extended schedules, or using different vaccines as the second booster.

  • That said, we can at least peek under the hood of those trials to look for clues.

  • Specifically, we need to look at what happened in participants between their first and second doses.

  • When Pfizer did that, they suggested that the first shot

  • was about 50 percent effective at stopping symptomatic cases of COVID-19,

  • though they cautioned against making conclusions based on this.

  • That's compared to about 95 percent after the second shot.

  • However, there's more than one way to look at data.

  • An analysis by the UK's expert committee on vaccines

  • suggests the first dose may actually be closer to 90 percent effective.

  • That's because they looked at the period between dose one and dose two,

  • but discarded the first two weeks of data,

  • since it's thought that the vaccine hasn't really kicked in yet at that point.

  • This does line up with what we think's going on biologically.

  • I mean, it takes time for immunity to ramp up.

  • But doing the math like this is controversial.

  • And other data, from research done in Israel, now suggests

  • the first dose of the BioNTech vaccine may not be as effective alone.

  • They're giving numbers around 33 percent, rather than 50 or 90 percent.

  • But their results are also controversial.

  • The data from the study hasn't been made publicly available or been peer-reviewed yet,

  • and it was an observational study, not a controlled trial.

  • Meanwhile, Moderna estimated the efficacy of their vaccine after just one dose was about 80 percent.

  • But they also cautioned that this data was based on small,

  • non-random samples and, like Pfizer, said it can't support a definite conclusion.

  • Both companies have said that they recommend

  • following the vaccine schedule supported by their respective clinical trials.

  • Now, there is the AstraZeneca vaccine as well.

  • And giving a second shot up to 12 weeks later is within their recommended dosing schedule.

  • Soone out of three ain't bad?

  • Complicating things, we also have no idea how long a single dose works for.

  • Like, Pfizer has said there wasn't evidence to support their vaccine being effective

  • if the second shot is delayed.

  • Again, the clinical trials just weren't set up that way.

  • One of the other huge unknowns around the UK and US's potential plans

  • is that they're allowing the booster shot to be a different vaccine, in extreme cases,

  • if the original isn't available.

  • Usually, when you go to the doctor and you get a booster shot,

  • it's just a second dose of the same thing.

  • Theoretically, the different vaccines are all training our bodies

  • to produce antibodies to the same virus, so it should work even if you mix and match.

  • Doctors will do this with some established vaccines if they have to, like if there's a shortage.

  • But the BioNTech and Moderna vaccines use a novel technology

  • and the mix-and-match approach hasn't been tested, so we don't know if it will work.

  • It might be that some subtle difference in the vaccines really does matter.

  • Some experts have even warned that the spaced-out schedule

  • could increase the risk of a vaccine-resistant strain of the virus.

  • Even for AstraZeneca, which did test a longer gap.

  • This is rare for human diseases, but it's possible,

  • for the same reasons bacteria can become resistant to antibiotics.

  • If people have enough antibodies to attack the virus,

  • but not enough to completely squash an infection,

  • that might select for mutations that get around the vaccine.

  • And if there are lots of people who aren't as well-protected thanks to single doses,

  • the odds of this happening increase.

  • So, overall, it is possible that delaying second doses of COVID vaccines

  • could spread protection to more people and slow the pandemic.

  • But in science, we can't say we know something if it hasn't been tested.

  • And there's one other important piece here.

  • See, ethics experts have raised serious concerns

  • about both the untested schedule, and mixing manufacturers.

  • That's because the people who participated in the clinical trials for these vaccines

  • benefited from the strict rules meant to protect them.

  • The experts are saying that suddenly changing how we give out the vaccines

  • is basically conducting an experiment without any of that protection.

  • And those experts are calling for actual testing before the UK, US, and others go off script.

  • Because the whole point here is to keep people safe.

  • So this is a gamble.

  • And it could pay offthe question is whether it's worth the risk.

  • Thanks for watching this episode of SciShow,

  • and thanks to our patrons for helping us make it happen.

  • Y'all are helping us take complicated, scary topics

  • and hopefully make them a little less scary and complicated.

  • And we could not do that work without you, so thanks.

  • If you'd like to help out, check out patreon.com/scishow.

  • [♪ OUTRO]

[♪ INTRO]

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B1 中級 美國腔

为什么一些国家推迟了 COVID 助推器注射(Why Some Countries Are Delaying COVID Booster Shots)

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    joey joey 發佈於 2021 年 06 月 04 日
影片單字