← More or Less
More or Less
How effective is one dose of the vaccine?
00:27:43
Is the first dose of the Pfizer Covid-19 vaccine 52% or 90% effective?
Loading summary
Transcript115 lines
- [00:01]
A
Hello and welcome to a brand new series of More or Less, where your weekly shot of statistical salvation that leaves you 90% less likely to be caught out by dubious data. And I'm Tim Harford. This week, did anything happen while we were off air? Fiddlesticks. Fine, I'll have a quick statistical catch up. The new vaccine regime is to delay the booster shot for up to three months. But is the first shot from 52% effective or 90% effective? We might have gotten away with it if it hadn't been for that pesky new virus variant. It's 70% more transmissible, apparently. But what does that mean? And one of our youngest loyal listeners has a question about her class register. But first, you don't need to look at the numbers to know things are bad. The kids are all off school, Joe Wicks is live streaming burpees and most terrifyingly of all, my producer Kate Lamble is back on More Or Less. Hello, Kate. You made More or Less throughout the first wave of the pandemic and now you're back and we're in national lockdown once again. And I have to say, it seems like rather too much of a coincidence, but look, how does the situation now compare to the situation back then?
- [01:17]
B
Yeah, so for more than 10 months now, we've had this constant stream of the numbers of COVID cases and deaths in the headlines. And sometimes, sometimes it can be quite hard to step back and realise what's been changing over time. So, to boil it down, I thought we could cover the current situation in five headline numbers.
- [01:34]
A
Good idea. Let's start with what last Saturday the BBC put out as a headline, UK sees highest daily toll of 1,325 deaths.
- [01:46]
B
So this article said 1,325 people had died in the UK, the biggest figure reported in a single day since the pandemic. Be. But the key word here is reported. There's often a delay between when a death occurs and when it's reported, especially when staff are overloaded. If we look at the date these deaths actually occurred, the highest figure so far during this second wave was on 6 January, when 799 people died within 28 days of receiving a positive COVID test.
- [02:19]
A
Is that higher than the spring?
- [02:21]
B
Well, the peak in the spring was the 8th of April, when there were 1,072 deaths. Thankfully, we haven't got there quite yet.
- [02:29]
A
Not yet. Ok, that's deaths headline figure number two. Then, after the most recent lockdown announcement, the government released a statement saying that one of the reasons for the new restrictions was the April 2020 hospital admissions peak has now been surpassed by 40%. Is that true?
- [02:47]
B
No. The Government has muddled up the definitions there. Hospital cases are 40% higher. They were over 30,000 when Boris Johnson gave his speech on 4 January and only around 21,500 in the April peak. Daily hospital admissions are up just 4%. If we compare the two weeks up to the 5th of January with the worst two weeks of the first wave,
- [03:11]
A
headline number three, then we often hear in the news that around this time of year hospital beds are full because of a rise in infections such as flu. But an intensive care doctor told BBC Breakfast this week to compare this to
- [03:26]
C
a normal winter flu epidemic is out of all proportion. It's orders of magnitude larger.
- [03:32]
B
This definitely isn't the same as a normal flu season. Data from Public Health England shows that in the worst recent year for winter flu, which was 2017, 2018, just under 10,000 patients were hospitalised over a 28 week period. That many Covid hospitalisations were recently registered in England in just three days.
- [03:54]
A
Wow. What about intensive care or high dependency units?
- [03:58]
B
There's a dramatic difference there too. In 2017 18, just under 3,500 flu patients needed to be cared for in an intensive care or high dependency unit over that 28 week period. But in a mere three weeks over the recent New Year period, just over 4,000 COVID patients were admitted to an ICU.
- [04:20]
A
So that's a bit more than 100 a week for flu and more than 1,000 a week for Covid. Let's move on to headline number four then. During Boris Johnson's most recent lockdown announcement, he.
- [04:31]
D
On 29th December, more than 80,000 people tested positive for Covid across the UK. A new record.
- [04:39]
B
Yes. Almost 81,500 samples taken on 29th December were positive for COVID 19. But since then, no other day has had that many cases. So far at least. Anyway. Sometimes tests do take a few days to be processed.
- [04:55]
A
If cases have been falling since then, does that mean 29 December was the peak of infection for this wave of the crisis?
- [05:02]
B
Not necessarily. It's possible that many people who felt ill around the Christmas period chose not to go out and get tested until the presents had been unwrapped and the turkey had been eaten. That's something we call a bank holiday effect. Honestly though, I can't be absolutely certain about that because according to government statistics, more PCR tests, those are the classic swab ones, were done on Christmas Day than 29 December, which is quite a way to spend your day. Basically, we probably need to wait a week or so until all the testing numbers go back to normal after Christmas to take a really good look at the case numbers.
- [05:39]
A
So, finally, headline number. On the same day, Margaret Keenan earned her pub quiz immortality by receiving the first dose dose of the Pfizer vaccine. At the start of December, the Health Secretary, Matt Hancock, was interviewed on the Today programme and he said, right now, several dozen people have been vaccinated and by Christmas, several million people will have been vaccinated. Millions of people vaccinated by Christmas. Did that happen?
- [06:09]
B
No.
- [06:10]
A
I feel like Charlie Brown trying to kick the football while Lucy holds it. Why on earth did I ever think that might happen? How many people were vaccinated?
- [06:18]
B
Well, across the UK, on 27th of December, there were just under 950,000 people who'd received the first dose of a vaccine.
- [06:26]
A
OK, so nearly a million. It could be worse. I heard the Prime Minister, Boris Johnson, during the latest lockdown announcement, say this.
- [06:34]
D
By the middle of February, if things go well, we expect to have offered the first vaccine dose to everyone in the four top priority groups identified by the Joint Committee on Vaccination and Immunisation. That means vaccinating all residents in a care home for older adults and their carers, everyone over the age of 70, all frontline health and social care workers and everyone who is clinically extremely vulnerable.
- [07:06]
B
Yes. So the Department of Health says across the UK, those four groups add up to 15 million people. As of 11 January, just under 2.3 million people had received at least one dose of a vaccine. So to meet Boris Johnson's target, by the middle of February, we need to be vaccinating, on average, two and a half million people a week. We're currently averaging less than half a million people a week. If we carry on at our current rate, it will take until July to reach all the groups Boris Johnson mentioned.
- [07:37]
A
It is reasonable that we could speed up vaccination as time goes on, though. The Oxford AstraZeneca vaccine wasn't available until the 4th of January. Should be a lot easier to distribute. In the past, though, Kate, you and I have seen the Government change the way they count things such as tests to hit Covid targets. One loyal listener, Adam Jacobs, heard the vaccine Minister, Nadim Zahawi, and he noticed this phrase.
- [08:01]
E
So we would have offered a vaccination
- [08:03]
D
to all those people to make sure
- [08:06]
B
that we can protect them.
- [08:09]
A
Vaccines offered, not vaccines injected. Surely they wouldn't do that. That would be like claiming tests had been carried out because a testing kit had been put in the post. It's Groundhog Day, isn't it? You and I are living out the plot of Groundhog Day. Then put your little hand in mine. There ain't no ill or mine notam.
- [08:30]
B
This is not Punxsutawney and you are not Bill Murray. We asked the Department of Health to specify what the target was and they confirmed the aim is to offer the vaccine to everyone over 70 or by February, not give it to them. They say that wording is being used because the vaccine is not compulsory. You can refuse it.
- [08:49]
A
I understand the point. If some over 70s don't want to take the vaccine, that's not the Government's fault. But what if on Valentine's Day, the government mails out 15 million letters offering the vaccine to people sometime in March or April? I mean, have people been offered the vaccine then? I don't want to sound like a cynic, Kate, but there is form here.
- [09:08]
B
We'll just have to wait and see. The good news is that More Or Less will still be on air, then, so we'll keep an eye out on the figures.
- [09:15]
A
Groundhog Day, I tell you. Then put your little hand in mine. Well, thank you, Kate. You're listening to More or less on BBC Radio 4 with me, Tim Harford. Now, my new side hustle is presenting a Radio 4 show called How To Vaccinate the World. But when loyal listener Andrew had a question about vaccines, he didn't write there. Instead, he wrote straight to More or less@BBC.co.uk because that is what being a loyal listener means.
- [09:44]
F
And he, my mum and her sister, both in their 90s, received their first dose of the Pfizer Biontech vaccine in the first week of its release. The effectiveness was then stated to be 52% after the first dose and 95% after the second. Politicians are now claiming that the first dose has an effectiveness of 90%. Can you find out who came up with the 90% figure?
- [10:07]
A
Well, Andrew, that figure of around 90% actually comes not from the politicians but. But from a statement by the UK Joint Committee on Vaccination and Immunisation, or jcvi, most of whom are scientists. To explain how they came up with this number, I spoke to Matt Keeling. He's a professor at the Mathematics Institute and the School of Life Sciences of Warwick University, where he studies epidemiology and vaccines. He is also one of the scientists who sits on the JCVI to work
- [10:38]
C
out Vaccine efficacy, you have a group of individuals who have been vaccinated, and you have a group of individuals that you've given the placebo to, and you monitor the level of disease in each. So as you follow these two arms of the vaccine wing forwards, you look at the differences between the two, and obviously, the day after you've had the vaccine, there's going to be very little difference. But as you go forward to two weeks, three weeks, you'd expect there to be a much bigger difference as the vaccine efficacy cuts in.
- [11:10]
A
So I'm just trying to think about this in terms of practical numbers. Maybe every week you've got 100 people in the placebo group who come down with COVID And if in the treatment group 100 people also came down with COVID you'd say, well, there's no evidence of efficacy. If 50 people came down with COVID you'd say, looks like efficacy is about 50%. If nobody came down with COVID you'd say, well, the efficacy seems to be close to 100%, maybe even 100%. But you're monitoring this week by week as the data comes in?
- [11:41]
C
Yeah, I mean, the actual monitoring is done day by day. And I believe the figure that Pfizer is quoting, they've been sort of quite pessimistic, and they've taken the efficacy, that is the reduction in the number of cases between the date when the first vaccine was given to the date when the second vaccine was given. And we know that certainly for the first week or 10 days, vaccine efficacy is very low. Your immune system hasn't been triggered, you don't know how to fight off the infection. And so, effectively, they're averaging over a period of very low vaccine efficacy and very high vaccine efficacy, and coming out with a figure of 52%.
- [12:21]
A
Interesting. So I think the delay that was measured in the trial is 21 days. So you're averaging from basically starting at zero efficacy, because you don't expect any efficacy in the first day. And then as you get closer and closer to the second dose being administered, there is at least a chance that you would see quite an efficacious vaccine. So what are the numbers suggesting, then?
- [12:43]
C
I believe the figure that's been quoted by JCVI is closer to 90%. And that's really because it's looked at the time scale from about 10 days after the first vaccine, where we know there has been a good immune response. It's a relatively short interval, but there's still enough data in there to actually measure how many fewer Cases there were within the vaccine group compared to those that receives the placebo.
- [13:08]
A
Well, yes, I did wonder about whether there was enough data because you're effectively discarding the first two weeks of data because you're not really expecting an immune response so early. But then if you've only got a few days before the second dose is administered, you might not have many cases.
- [13:25]
C
I mean, there's a sufficient number of cases within that interval, you can probably extend it slightly and say, well, we know that the second dose isn't going to have an immediate impact, so you could probably go a week after the second dose. But that gives very, very similar sorts of numbers. You're still talking in the high 80s as vaccine efficacy.
- [13:44]
A
The suggestion then is the efficacy is pretty high after the first dose. You're not relying on the second dose to make the vaccine work. But do we have any data on what then happens? So we know what happens after three weeks, but we don't know what happens after three months. For example, for all we know, efficacy could be very high or it might drop off a cliff if you don't have the second dose. Are we just drawing on experience from other vaccines or do we actually have any data about what would happen if you didn't administer the second dose immediately?
- [14:16]
C
So you can read a lot of information between different vaccines. There's been a lot of detailed studies of the Moderna vaccine and the Oxford AstraZeneca vaccine, where we can actually see how immunity within individuals drops over reasonable timescales. There's absolutely no indication that we'd expect vaccine efficacy to suddenly fall off a cliff at three weeks. The immune system just doesn't work like that. You get slow waning over time, but we don't even think that's going to happen on the 12 week timescales that we're talking about.
- [14:50]
A
Matt Keeling, if you want to hear more about the decision to leave 12 weeks rather than 21 days between vaccine doses, you might be interested in this week's episode of my other Radio 4 gig, how to vaccinate the World, which you can hear on BBC Sounds, or you can tune in on Monday mornings at 11:30. One of the justifications for the new lockdown measures has been the appearance of a new variant of the virus that's reported to spread more easily. This is the Prime Minister, Boris Johnson, speaking at the government press conference on December 19, when it was announced that Christmas gatherings would be curtailed across much of the country.
- [15:34]
D
Although there's considerable uncertainty, it may be up to 70% more transmissible than the old variant, the original version of the disease. This is early data and it's subject
- [15:47]
A
to review, but since then, this figure that the new variant is 70% more transmissible has been repeated endlessly.
- [15:55]
C
This new variant appears to be up
- [15:57]
A
to 70% more transmissible.
- [16:01]
B
It's up to 70%. 70% more transmissible.
- [16:04]
A
And we've had a number of loyal listeners email more or lessbc.co.uk to ask, how have experts worked that out? Well, 10 year, more or less. Veteran Charlotte McDonald is here to tell us more. Hello, Charlotte.
- [16:18]
E
Hi, Tim.
- [16:20]
A
So where does this 70% figure come from?
- [16:22]
E
The short answer is it came from research headed by this man.
- [16:26]
G
My name is Dr. Eric Voltz, I work at Imperial College London in the Department of Infectious Disease Epidemiology.
- [16:33]
E
He's constantly being asked where the 70% number comes from, but this is a world exclusive. It's the first time he's spoken on air.
- [16:41]
A
Well, tell us more.
- [16:42]
E
You know, Tim, it turns out that the UK is actually pretty good at regularly collecting samples of the virus across the country for full RNA sequencing. Viruses naturally evolve and mutate as they move through a population, and so virologists keep an eye on changes to the genetic code of the virus, which could change how it behaves, mutations which could allow it to infect cells more easily. For example. Now, when people are tested for Covid, around 10% of samples that have proven positive for the virus undergo RNA sequencing. That's thousands of samples every day. This data is made available to scientists, allowing Dr. Voltz to track the different genetic variants, also known as lineages, of the virus. In December, Dr. Waltz was made aware of a particular mutation that was causing concern to scientists abroad.
- [17:31]
G
When we looked at whether this was present in our own database, we found that it wasn't only present, but it also occurred on this particular lineage, which carried with it a whole bunch of other interesting mutations. And that's really what triggered the initial alarm that this might be something unusual. The next step was for an analyst like me, in this case, to dig through the data and see how quickly the virus was spreading and what parts of the UK it was spreading in.
- [17:58]
E
Then, doing some mathematical modelling, he and colleagues were able to work out how this variant, named B117, was spreading.
- [18:07]
G
It's a bit like comparing, like a horse race and you want to know which horse is likely to take the lead. What we can do is look at the frequency that this new variant appears over time and how that changes and the rate. In this case, B117 is increasing tells us something about how much more transmissible it's likely to be in the population.
- [18:29]
E
And what they found was that the new variant is 70% more transmissible than the variants present in the UK already.
- [18:38]
G
We've been tracking lineages for many months now and that value of 70% was much bigger than anything we had seen before.
- [18:46]
A
Okay, so essentially this 70% number has been calculated by plotting the spread of the many different genetic variants of the virus from samples taken across the country and comparing how fast they're spreading. But what confuses me, Charlotte, is what exactly does it mean to say it's 70% more transmissible?
- [19:06]
E
Well, this is how Eric explains it.
- [19:08]
G
If we take this difference in growth to be basically like a multiplier times the reproduction number, that basically means someone with the old variant might infect one person, then someone with the new variant would infect 1.7 people on average.
- [19:23]
E
So if you had R of 1 with the old variant, with 10 cases now leading to 10 new cases in the future, now you'd have R of 1.7 with 10 cases leading to 17 new cases. Now we know that to have an R of only one, you need restrictions and social distancing. So if we weren't taking any precautions at all and had no immunity, R might rise from about 3 to about 5.
- [19:50]
A
Wow, these are big numbers.
- [19:51]
E
They are. And this isn't just mathematical modelling anymore. Eric says they saw a clear correlation in areas where Covid cases were rising sharply. Cases of the new variant were also up. This all happened very quickly. On December 10, Dr. Folts first reported his findings to NervTag, which all too many people now know stands for the New and Emerging Respiratory Virus threats advisory group. Nine days later, on December 19, Boris Johnson was using the stat in a press conference.
- [20:23]
G
Overall, I think we caught it quite early because when we wrote that first report, there were only about 900 genomes that had this variant. And that might sound like a lot, but it was on a background of approximately 60,000 that had been collected over the autumn.
- [20:39]
A
So this work was done quickly. And looking at a relatively small number of genome samples, have there been any other studies since then that support these conclusions?
- [20:48]
E
Yes, there has been a lot more data available to analyse since Dr. Waltz's first estimates. Dr. Rosie Barnard is a research Fellow in Infectious Disease Modelling at the London School of Hygiene and Tropical Medicine. She co authored a different report into the spread of the new variant.
- [21:04]
B
We can see that the proportion of the new variant is growing in time.
- [21:08]
H
Then we started to look at different
- [21:10]
B
explanations behind why that might be happening.
- [21:13]
H
And the best mechanism that we found,
- [21:16]
B
fitting the data was increased transmissibility of the new variant.
- [21:21]
H
Those estimates we found between 50 and 74% increased transmissibility.
- [21:27]
A
So we've established that it is more transmissible, but at least it seems not to be more deadly.
- [21:32]
E
Yes, that's true. Although, ironically, if it had been 70% more deadly rather than transmissible, it might have been a less worrying variant.
- [21:40]
A
Explain.
- [21:41]
E
So imagine a virus with an R number of one, so each person who's infected infects one more person. Let's say that you start with 10 cases and each week you get 10 new cases. Now, instead, imagine the variant that's 70% more infectious, so R is 1.7. In the second week you get 17 new cases. In the third week there's 29. By week 10, you have nearly 1,200 new cases instead of 10. More than 100 times as many new cases. And all things equal, more than 100 times as many deaths.
- [22:15]
A
So that's potentially much more serious than having a virus that's 70% more deadly. Right.
- [22:21]
D
Gulp.
- [22:22]
A
Thank you, Charlotte. You're listening to More or less on BBC Radio 4 with me, Tim Harford. Now, while we've been preoccupied with vaccines and testing, we received a message in December from a young family with a very different question.
- [22:40]
C
Hi, More or less, this is Lawrence in Cambridge and I'm here with Madeline and Madeleine. Have you been writing Christmas cards to your school friends?
- [22:50]
H
Yes.
- [22:51]
C
How many children are there in your whole class? 28. So there's 28 children. And in writing our Christmas cards, how many did we find started with the letter A? 10. That is a lot of the names beginning with the letter A. So what we were wondering, more or less, is how uncommon is this?
- [23:17]
B
Yes.
- [23:18]
A
Well, how could we refuse? And Madeleine's story checks out as far as I'm concerned. One third of my children have a name that starts with A. A very similar proportion to Madeleine's class. But let's go deeper. I asked our chief name correspondent, Maya Loison, to look into it. Hello, Maya.
- [23:35]
H
Hi, Tim.
- [23:36]
A
So, what have you found?
- [23:37]
H
So I contacted our friends at the Open University who were more than happy to help. Dr. Khosub Adhikari is a lecturer in statistics and I asked him, if we assume each letter of the Alphabet is equally probable, then what are the chances? 10 out of 28 names in Madeleine's class begin with the letter A.
- [23:55]
F
So if we assume that each letter is equally probable, then each letter has a 1 out of 26 chance, which is about 3%. So with that chance of about 3%, we can actually calculate the probability and see that it is very rare.
- [24:12]
A
Well, how rare?
- [24:13]
H
It roughly comes out as a 1 in 21 million chance of seeing something like that.
- [24:18]
A
Wow, that's win the lottery sort of odds. If the letters are equally likely to be initials, but of course they aren't. There are lots of A's. There aren't many Z's. For example, I wanted to name my son Zod, but my wife vetoed it.
- [24:32]
H
Really, Tim, I would have thought you'd gone for a more normal name like Neil before Zod.
- [24:36]
A
Now that is an idea. But the point is, Maya, knowing that A is a much more common initial, how does that change the calculation?
- [24:44]
H
I wondered the same thing. Costup has looked into the official data on the most popular names of children born in 2015. As we are looking at reception age children here, it's based on about 600,000 births. He found that A is in fact the most popular letter for girls names.
- [25:02]
F
About 46,000 baby girls had a name starting with A. So that's about 15% of all the names. So you can see that the name A is really, really popular.
- [25:14]
A
Don't keep us in suspense. What is the most popular name for girls beginning with A?
- [25:20]
H
It's the name Amelia, which is not only the most popular A name, it's the most popular girl's name. Full stop. There were 5,000Amelias born in 2015. Ava also made the top five. And other popular names include Alice, Abigail, Amber and Amelie.
- [25:36]
A
And what about boys?
- [25:37]
H
Amongst boys, J is the most popular starting letter. Jack and Jacob are in the top 10, for example. But A is still more popular than most letters.
- [25:46]
F
So J is seen in about 40,000 boys and a is seen in about 35,000 boys. So that proportion of A in boys comes down to about 10%. So it's a bit less common than girls, but it's still not that uncommon.
- [26:04]
H
The most popular boy's name beginning with an A was alfie, which ranked 14th. There were about 2,500 Alphys. And also in the top 50 were Alexander, Arthur and Adam.
- [26:17]
A
Okay, so let's crunch the numbers. What are the chances of having 10 children with names starting with A in a class of 28 kids?
- [26:26]
H
Causteb did the sums and says it's a 1 in 780 chance.
- [26:30]
A
We know in Madeleine's class there are 28 children, 15 girls, 13 boys. So taking that as a given, what's the chance we see exactly seven girls and three boys with first names beginning with A.
- [26:43]
H
The chance of this particular scenario in that class is about one in 3,000. We know that there are 18,000 schools in England and Wales. If every class was the same size and gender breakdown as Madeleine's, we'd expect just six of them to have seven girls and three boys with names beginning with A.
- [27:01]
A
Well, that's exciting.
- [27:03]
C
Even more exciting with Is it more exciting than Christmas?
- [27:10]
A
No. Yes, it is. It's more. No, it's not more exciting than Christmas. Thank you to Madeleine for her question. You're welcome on Radio 4 anytime, Madeleine. Thanks to Kostab Adhikari of the Open University for his answer and thanks to our own Maya Loison. And if you have a question for us, as Madeleine did, please email more or lessbc.co.uk we will be back next week. Until then, goodbye.