What does it mean that a Covid vaccine is, say, 70% effective?

Let’s start by talking about what it doesn’t mean. If a vaccine is 70% effective against a virus it doesn’t mean that 30% of the people who’ve been vaccinated will get infected. That’s an assumption that only people whose math is as bad as mine would make. The kind of people who juggle the numbers 70, 30, and 100 and come up with an answer that’s as likely looking as it is meaningless.

It turns out that the number doesn’t compare the vaccinated people who stay well to the  vaccinated people who get sick. Nay, verily, it compares the vaccinated people who get sick (or who stay well) to the unvaccinated people who get sick (or who stay well). To put that a different way (because I don’t know about you, but I’m struggling with this), it compares the risk vaccinated people run in the presence of Covid to the risk unvaccinated people run.

This matters because it leaves us with a much smaller pool of people who are vulnerable.

If you’d care to read about that with actual numbers and sensible writers, follow the link. I have a severe allergy to math and I know better than to attempt a full explanation. 

Irrelevant photo: Red clover. I’ll come back with more kitten photos soon.


Who gets sick with the Delta variant and can the vaccinated spread it?

In the US, 95% to 98% of the people hospitalized with Covid are unvaccinated, and 99.5% of the deaths are of the unvaccinated. Even with the Delta variant circulating, that seems to be holding true.

But some numbers have changed since the initial vaccine studies, and they have to do with what Dr. Robert Schooley, from the Department of Medicine at UC San Diego’s School of Medicine, calls “the asymptomatic shedding rate among vaccinated individuals,” which in human speech means how much of the virus is spread by vaccinated people who get infected but don’t get sick.

Or to push that one step further, how dangerous they are to the unvaccinated.

When the vaccines were tested, the Delta variant wasn’t around yet. They were dealing with a less infectious beast. On top of which, no one thought to investigate the odd sniffles and colds that people in the study cooked up. They were allergies. They were colds. They were flu. 

Remember the old days, when people caught colds? 

So the study didn’t track them.

Now, though, they’re realizing that those mild symptoms could be nearly asymptomatic Covid, and what’s known so far is that some fully vaccinated people who get infected carry enough of the virus to spread it, even though it’s not making them sick.

How do they compare with unvaccinated people as far as spreading the thing goes? The numbers aren’t in yet. I mean, they’re out there. Numbers always are. But nobody’s assembled them yet. On average, though, Schooley says the infected vaccinated person will shed less virus for a shorter time. And the odds that they’ll become infected are lower, so whatever the eventual picture turns out to be, vaccinating people does slow the spread of the disease.


Are the vaccines losing their effectiveness? 

As has become usual since the pandemic started, we’re not likely to find a definitive answer yet, but it does look like the number of breakthrough cases in vaccinated people is growing.

What’s a breakthrough case? A Covid case in someone who’s vaccinated. Getting one doesn’t mean you’re dead, hospitalized, or even necessarily sick. It just means you’re carrying the infection, when if the vaccines were 100% effective (very few are and no one expected these to be), you wouldn’t be. 

So if you’re fully vaccinated, it’s not time to panic yet. You can always do that later. 

Why’s this happening? The experts are still debating that, but it doesn’t look like the Delta variant is evading either the vaccines or immunity from earlier infections. 

That’s another reason to wait before you panic.

If Delta hasn’t broken through the vaccines’ protective lines, that leaves us with two possibilities. One, the vaccines’ effectiveness is fading, or two, Delta’s high transmissibility is responsible.

Several studies show what could be a waning in vaccine effectiveness, but it’s hard to know if the numbers really mean that. They could also mean that vaccinated people are taking more risks–going to bars or gyms or other Covid exchange sites–and giving themselves more chances to meet the virus.

And protection against getting so sick that you need to be hospitalized, though, is holding steady, which may mean effectiveness isn’t waning. All this will be perfectly clear in hindsight, but for now we have to make do with what we can see from where we are.

So do booster shots make sense? 

On the side of saying no are the many countries that the vaccines have barely reached. How can rich countries be talking about booster shots when initial doses are desperately needed elsewhere?

On the side of saying yes is that in people with weakened immune systems, because of either age or disease, they can make a difference, although the evidence on that is still preliminary.


Testing news

A new study shows that testing saliva for Covid is as reliable as testing nasal swabs. So at some point we may be able to stop puncturing our brain pans with sticks that are allegedly softened with cotton wool–or something that looks vaguely like cotton wool.

If Covid tests shift to using saliva, they won’t have to rely on patents’ willingness to make themselves uncomfortable, which will make them more reliable. And we won’t have to worry about a shortage of swabs.

If, in fact, worrying about that is one of the things that’s keeping you up at night.


Several times now, I’ve sworn off writing about newer, faster, cheaper Covid tests because although I keep reading about them, they never seem to be adopted–at least not anywhere I read about, and certainly not where I live. But you know how it is when you swear off something multiple times: It’s a sign that you keep breaking your word. So here we go again:

A newer, faster, cheaper Covid test has been developed. And it uses the same stuff that pencil lead is made from, which isn’t lead at all, it’s graphite. It cuts the cost to $1.50 per test, takes six and a half minutes, and is 100% accurate using a saliva sample and 88% using one of those evil nasal thingies.

The system can be adapted to test for other transmissible diseases. Now all we have to do is wait and see whether we hear of it again.

69 thoughts on “What does it mean that a Covid vaccine is, say, 70% effective?

    • Thanks, Jean. We’ve been coping with a sick dog (not to mention the kitten) and I’ve been too tired to write much. I really should be working on a book manuscript, trying to unscrambled a scrambled but I think promising mess, but unfortunately blogging has this addictive quality–

      Liked by 1 person

  1. So much to worry about again.
    I had also wondered about booster shots when so much of the world has not had even one jab.
    Americans don’t seem to be as altruistic as we thought we were. Aren’t you glad you moved to Britain before the Afghanistan departure?

    Liked by 1 person

  2. That new test sounds great. I hope it does get adopted but, like you, I have noted that a lot of new developments in the fight against the pandemic never seem to make any progress or gain traction – at least not that I am aware of.

    I cannot believe that we are discussing booster shots for a couple of reasons. First of all, despite being a pessimist, I really didn’t anticipate that we would feel almost back to square one at this point in 2021 after several months of vaccine roll outs. Friends who work in relevant healthcare departments are reporting that things look and feel worse now than they did last winter, partly because they are seeing a higher proportion of much younger people on ventilators than before and partly because so many of these hospitalizations could be prevented through vaccination so it takes a mental toll.

    The other reason I have qualms about booster shots is precisely the one you mention about the injustice of some regions of the world deploying resources to give a third shot when other regions are struggling with their roll out. We cannot get ahead of this pandemic until we suppress the circumstances that would create more and worse variants. However, I am a hypocrite, because I will get my booster shot when eligible given that I work with a vulnerable population not yet eligible for the vaccine.

    Liked by 1 person

    • I struggled to write something about the higher proportion of young people who are seriously ill but bogged down in the numbers. It sounds like no one’s clear yet what’s happening, but some people think the higher proportion of young people on ventilators is a function of the higher proportion of older people who are vaccinated.

      Let’s hope they’re right.

      I can’t imagine how people who work in healthcare these days are holding themselves together. My hat’s off to your friends.

      Liked by 1 person

  3. Here in Portugal we are more than 70% fully vaccinated and our covid rates are falling dramatically, not just because of the vaccines but because we still have mask mandates in all indoor paces and outdoors in close contact. That is what is missing most in the US, people get the vaccine, and then mask mandates are dropped (like Florida and Texas) so the virus continues to circulate. we must keep wearing masks until covid is no longer a threat!

    Liked by 2 people

  4. Hope the dog is better! Our next door neighbour’s Dad had a breakthrough covid and ended up in hospital, but has recovered well thankfully. I’m noticing more and more people are not wearing masks at all now, sigh. We’ve still got a long way to go.

    Liked by 1 person

  5. My mum and dad have both just had the virus, despite being fully vaccinated. It seems likely that Dad got it first and passed it on to Mum, but we’ve no idea where he got it. He’s 76: he hasn’t been hanging around in crowded nightclubs. Three friends with whom they’d spent a few hours then also tested positive. All fully vaccinated, so for all 5 of them to have got it is pretty worrying. They’re all in their 70s, so were all vaccinated months ago, so it does seem that the efficacy of the vaccines is wearing off. Thankfully, none of them have been seriously ill with it.

    Liked by 2 people

    • A vaccinated older person around here (translation–he’s my age) just came down with it, although I don’t know at this point how serious it is. What you’re saying is sobering. I guess the central issue is how seriously ill they get. Fingers crossed and all best wishes.

      Liked by 1 person

        • I’ve been reading about the Boardmasters music and surfing festival that just ended here in Cornwall. Despite assorted precautions (and because of crowding and alcohol) something like a tenth of the people who attended have tested positive. We seem to be hell-bent to undo the progress we’d made.


  6. Good summary of what info is available. I can’t add anything. I will be able to get a booster in October (6 weeks after my second shot) and I have a doctor’s appointment about then. A neighbor – who is 20 years younger and also has COPD and multiple myeloma – has understandably gotten her booster.

    Hopefully the UK is not succumbing to the temptation to use the veterinary wormer Ivermectin, as some US fans of Faux News have done.


    Liked by 1 person

    • I don’t think we’ve gone quite crazy enough to mistake ourselves for horses or whatever yet. Or at least, our insanity runs in a different direction–but let’s not talk about Brexit just now.

      You stay well, okay? Whatever it takes.


  7. Hey Ellen. To me it’s all as clear as mud. My brain is overloaded and full. It doesn’t want anymore COVID news or stuff. I’m fully vaccinated. In BC were at around 76% but now they dropped mask mandates then reinstated as numbers escalated. Fights ensuing over ignorance and intro of vac-passports. I’m going back to my cave. 🇨🇦🤯😷

    Liked by 1 person

  8. Basically it’s a virus related to the common cold, and they haven’t found a vaccine for that one. You can catch Covid even if you have been vaccinated. I think it all boils down to staying away from lots of people that are in the same room. It’s here to stay and we have to learn to live with it like we do the common cold.

    Liked by 1 person

    • But since it’s a lot more dangerous than the common cold, learning to live with it has to involve sensible things like masks, ventilation (we’re barely talking about that yet), and as you said, avoiding crowds at least until we get a better handle on it.

      Part of the problem in discussing it is that people mean very different things when they talk about learning to live with it.

      Liked by 2 people

  9. “They could also mean that vaccinated people are taking more risks–going to bars or gyms or other Covid exchange sites–and giving themselves more chances to meet the virus.” I think this is what happens in most cases. Most, not all.

    Liked by 1 person

  10. Even my notoriously cautious other half planned to venture to an outdoor event, but the news about the fallout after that local surfing event in your neck of the woods has made him change his mind. I’m scheduled to take my mother to the wake of an old family friend who died during lockdown, and I know he’s looking askance at me, wondering what germs I’ll bring back with me. I’m wondering about the acceptability of wearing a mask throughout it, and where I can get a stock of higher protection level masks in time. I enjoyed catching up with the grandbugs recently following their return from 5 weeks holiday in Spain and after their parents had assaulted their nasal passages with multiple tests, then worried about whether I had COVID and could be guilty of passing it on to them – not because I had any symptoms, but because I’d not tested myself and could be asymptomatic.

    I suspect we’re all worried out, but there’s no clear end in sight, so we’ll have to keep on keeping on.

    Best wishes to the poorly pooch.

    Liked by 1 person

    • Thanks, Deb. We’re seeing a response from one leg, so there’s hope. Fingers crossed.

      We have a memorial coming up in the fall, which I’ll admit worries me. For what it’s worth, though, I’m told the that Boardmasters event included a lot of things happening inside tents. And probably lots of people inside the local pubs and people packed into any space they could find. And photos of the outdoor events make them look like people were practically sitting in each other’s pockets. So I’m not sure it’s a fair judgement on (smaller, saner) outdoor gatherings.

      Liked by 1 person

      • Oh that sounds way more serious that “poorly”. I upgrade my simple best wishes to a ton of ’em, and add some healing vibes and any other magic which may do the trick.

        I suspect the military history event wouldn’t be quite as – ahem – friendly as the Boardmasters but, from the evidence of previous years, there will a fair bit of all-day drinking. On top of which there are generally a goodly number of “characters” in attendance and exhibiting, which he fears may equate to anti-vax views. He could be wrong, but he’s not certain the event is worth the risk, especially as he’d previously mentally written it off till next year.

        Liked by 1 person

  11. The other day I had what NPR calls a “driveway moment” – you know, when you’re listening to one of their programs while driving and when you get home it’s still on and you’re so engrossed that you turn off your engine but stay in your car to keep listening. Anyway, I was having one of those because they were interviewing some epidemiologist about COVID. He said something I hadn’t heard before. He said people (including the experts) were incorrectly defining “breakthrough infection.” He said a breakthrough is not when a vaccinated person gets an asymptomatic or mild COVID infection, but rather when a vaccinated person has to be hospitalized with COVID.” Hmmm…
    Then, the very next day on NPR, on a different show with different experts, it was back to the old definition. Maybe it’s all just a semantics game and doesn’t really matter, but I find it just another way this whole thing is confusing.

    Liked by 1 person

  12. I read a news article this morning where a bloke spent 6 days in a medically induced coma because he’d decided to wait and see what the vaccine side-effects were before getting a shot, and of course, caught the virus. (thankfully he survived and has had a shot) He now says he wished he’d got vaccinated earlier when he had the chance.
    I have no more fucks to give.

    Liked by 1 person

    • I wish someone would gather those folks together for a series of ads/press conferences/whatevers. Not that the hard core antivaxxers would believe them, but the more people who get peeled away the better. But yeah, I understand what you’re saying about running out of fucks.

      Liked by 1 person

  13. My niece is a nurse in ICU in the West Country and it’s been very hard going for her – all the covid patients are unvaccinated people and everyone (not the nursing staff obviously, just the families) are very surprised when they die beacuse they are relatively young…Anti-vaxxers have a lot to answer for.

    Liked by 1 person

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