Can someone explain the vaccine efficacy to me

[doireallyneedaname]

What does it actually mean if it’s, say, 60%?

Does that mean 60% of people didn’t get covid? Or didn’t display symptoms?

Does it mean the other percentage DO get covid as normal, and may have severe symptoms? So the vaccine just doesn’t work for them?

If you get the vaccine, are you guaranteed to not suffer severely from Covid?

Vaccine efficacy/effectiveness is interpreted as the proportionate reduction in disease among the vaccinated group. So a VE of 90% indicates a 90% reduction in disease occurrence among the vaccinated group, or a 90% reduction from the number of cases you would expect if they have not been vaccinated (after the period immunity kicks in).

The VE numbers for COVID-19 you see quoted measure symptomatic covid (irrespective of the severity of symptoms).

e.g. the VE rates quoted for the AstraZeneca/Oxford vaccine of between 62% and 90% is for people getting any symptomatic covid at all (after a 3 week period).

It was reported that no one in the trail having AstraZeneca/Oxford was hospitalized and only 1 person had severe symptoms (but treated at home).

Thanks. Does that mean the remaining percentage did get symptomatic Covid? I understand nobody in the Oxford trial required hospitalisation but what about other vaccines? Sorry, just trying to get my head around it in layman’s terms.

It’s down to individual person, symptoms when getting covid should be 60% less severe, than not having the vaccine.

I dont understand it either. Even with the explanations.

Does it mean if 100 people have the vaccine andcare exposed to the virus. 60 wont get it and 40 will but not as badly.

Or if 100 people were vaccinated and exposed to the virus, they would catch it but 60 of them wouldnt have symptoms and 40 would have mild toneeding treatment

Or something else?. Is it meanat an individual level id have 60% chance of not catching it . Argh. Why cant i understand this .

You need to look at the endpoints for the pivotal trials. You can't compare Oxford vs Pfizer, the trials are not directly comparable due to their endpoints.

There's a good podcast on Radio 4 that explains this - More or Less, 13th January. www.bbc.co.uk/sounds/play/m000r4t9

Vaccines are tested with a control group. So there are two groups, both receive an injection and don't know whether they've had the vaccine or a placebo. Then both groups are tracked to observe outcomes. 90% efficacy means that in the vaccinated group, 90% fewer got covid. Eg in the control group 100 people got it whereas in the vaccinated group 10 people got it.

So ... the vaccine stops you from getting covid? I thought it meant you could still have it but it massively reduced symptoms. You could still carry it and spread it .... I feel like I have zero actual understanding of this too despite reading quite a bit over the last few months.

I guess my question is this:

If you get the jab, are you guaranteed to not suffer severe Covid?

It means that if you had twp populations of 2000 people, one vaccinated and one not, and they were all exposure see to covid, the %age protection is the difference in numbers of cases in each group.

So for 90% you could (for example and I'm taking an attack rate at random just for ease of making the point) have the difference between

• unvaccinated, half become ill, so 1000 cases with some serious cases and a fatality
• vaccinated, case numbers drop by 90% so 100 become ill with no severe cases or fatalities

It could be that in some people the vaccine does not 'take' (ie the expected immune response does not form). There would be no way of knowing if this had happened to you, so catching it remains a real possibility. And serious cases and deaths will still happen, but in far lower numbers. As Vallance said, it's looking for the acceptable level of risk to society, not perfect protection for every individual

If you get the jab, are you guaranteed to not suffer severe Covid?

No

There are never any 100% guarantees when you're in biological systems. Anyone who claims there is, is a charlatan. Anyone who thinks there should be is misguided (or in the case of the Russian antivax troll factories - see WHO reports on their existence and tactics - pushing deliberate distortion)

No you can never have a 100% guarantee. It's just a lot less likely.

However, it works in a population level because if 60% of people getting the vaccine no longer gets covid, then you have a potential 60% reduction of cases.

On an individual level, there's no guarantee you'll not be the 40% that either have mild covid or didn't have a good enough immune response at all.

It's even better at the population level if that 60% are no longer carriers. That will heavily reduce the R number and protects the other 40%. I believe they say they needed about 70% or so to get herd immunity.

I found this great video that both explains how the three possible current vaccines we have work, the trial stages and the efficacy/prevention of disease aspects. Worth watching.

Different endpoints in the Pfizer / Oxford trials, which are you wanting to know about?

@scaevola - aaah. I think i understand that explanation. Thank you

[quote Choconuttolata]I found this great video that both explains how the three possible current vaccines we have work, the trial stages and the efficacy/prevention of disease aspects. Worth watching.

[/quote] That was a very interesting watch. Thank you for sharing 😊

The chart from the Pfizer trials (which has been shown on many other MN threads) clearly shows that it doesn't totally stop cases of Coronavirus, but they are MASSIVELY reduced.

There are many different types of vaccine efficacy - efficacy against infection, against transmission, against disease, and against severe disease - and these can vary for a single vaccine. How are they related?
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