1/ Thanks for this thread on the paper that came out today, Meg! @kallmemeg
threadreaderapp.com/thread/1460205…
threadreaderapp.com/thread/1460205…
2/ I guess it will take a lot longer - and a different sort of study - to estimate the proportion of asymptomatic infections it will prevent. Until we know how many people are prevented from being infectious, it's hard to know what effect three doses will have on transmission.
3/ BTW, can we please stop calling the third dose a "booster dose"? I'm pretty certain that this will eventually become a three-dose primary course (with subsequent reinforcing doses if necessary, eg if there's waning of immunity or if there are vaccine escape variants.)
4/ Having been asked, earlier, what effect a third dose will have on transmission I wrote something along the following lines…
5/ You reach herd immunity when fewer than one of the people you would have infected is still susceptible to infection. This means that they are unable to be infected and infectious.
6/ (NB - to be infected and infectious, not to be symptomatic. What really matters is the proportion of the secondary cases who will, in turn, be infectious and able to generate tertiary cases.)
R0 (subscript zero) is a theoretical measure of how infectious a disease is.
R0 (subscript zero) is a theoretical measure of how infectious a disease is.
7/ It is the number of people which (on average) each case will infect. It is pronounced "R-nought" (or, if you're American, "R-naught".)
8/ Of course "on average" is doing a lot of heavy lifting here. Some people have a lot more contacts than others. Children and teenagers tend to have very high numbers of contacts; retired people probably relatively few (on average).
9/ And people who have more contacts will infect more people...
10/ It's also the case that the number of people each person infects is far more variable for Covid-19 than it is for other diseases, such as flu...
11/ For the delta variant of SARS-CoV-2 virus, R0 is thought to be somewhere between 5 and 10.
health-desk.org/articles/how-c…
health-desk.org/articles/how-c…
12/ But to keep it simple, let's pretend everybody is equally likely to be infected/infectious. And for the ease of the arithmetic, let's pretend Covid's R0 is 10...
13/ That being the case, to achieve herd immunity, you'd have to get immunity rates >90%. That way, more than 9 of the people 10 people you would otherwise have infected will be immune, unable to be infected.
14/ If the disease has an R0 of 5, then you'd need to ensure >4 out of every five people are immune - an immunity rate of >80%.
15/ Of course, it's more complicated than that. What's the R value for people who have had one, two or three doses - how many people will they (on average) go on to infect? It's likely that people will be less likely to be infected/infectious...
16/ And it's more important, if your goal is herd immunity, to get the people who have the most contacts with other people immune, unable to be infected and infect others...
18/ I estimated that, after two doses of vaccine, the chance of having any infection is likely to be about 30% what it would if unvaccinated - so the maximum proportion of the population you could get immune and unable to pass the disease on is about 70%.
19/ If R0 is as high as 5 - let alone as high as 9 or 10 - you will never achieve herd immunity, even if 100% of the population is vaccinated with two doses of vaccine.
But after three doses, I estimated that only about 3% of the population can be infected.
But after three doses, I estimated that only about 3% of the population can be infected.
20/ If you gave everybody three doses of the vaccine, you'd have 97% immunity - more than enough for herd immunity, even if R0 is 10.
On this basis, it seems unlikely that we will achieve herd immunity with a two-dose vaccination regime.
On this basis, it seems unlikely that we will achieve herd immunity with a two-dose vaccination regime.
21/ But if we can get enough people vaccinated with three doses - and especially, enough of the people who have a lot of contact with other people - we would be able to achieve population immunity.
22/ (I have been predicting, confidently, for some time, that the vaccines will become a three-dose vaccination programme, like so many of our other vaccine programmes: a three doses "primary course".
23/ This primary course may subsequently be followed a year or more later by [a] further reinforcing dose[s], if immunity wanes, or if vaccine-escape variants arise.)
24/ But - especially since children and young people tend to have a lot of contacts - we don't have a chance of achieving herd immunity and controlling the virus with vaccination alone, unless we can give a large proportion of children and young people three doses of vaccine.
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