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Yet another post about COVID19 vaccines. Ironically I started with Moderna for the first two posts then shifted to Pfizer for the 3rd post when they released data first. Yesterday Moderna actual data became available (great news), along with some additional information we can model. Literally as I was writing the post more Pfizer data came out with even more good news.
Last time
Last time we paid homage to Doing Bayesian Data Analysis by Dr. John K. Kruschke (hereafter, simply DBDA) while exploring overall effectiveness of the Pfizer vaccine.
This time we’ll revisit brms
and STAN
which we touched upon in part 2. At the time I wrote:
Quite frankly it is more than we need for our simple little analysis. A bit like using a sledge hammer as a flyswatter. Besides the simple joy of learning about it however, there is one practical reason to use it. As our data grows and we inevitably want to conduct more sophisticated analysis factoring things in like gender, age and race. It is likely to become the tool of choice.
Since the Pfizer data includes some early glimpses at information about factors
such as age and race we can put brms
to much better use this time. First step
let’s load some packages.
library(dplyr) library(brms) library(ggplot2) library(kableExtra) theme_set(theme_bw())
What we know. Approximately 25,650 in the study (well at least eligible so far). Two age categories (Less than 65, or older than that). We can safely assume they are not evenly distributed across age groups, but are more or less equal in the number who received the true vaccine versus the placebo. 95 subjects contracted COVID, the rest did not. Of those 95 only 5 received the true vaccine yielding an effectiveness of ~ 94%. We don’t know from the press releases what the proportion by age was. But we can have some fun playing with the numbers and preparing for when we see fuller data.
One area of interest will no doubt be whether the vaccine protects older people as well as younger. So we’ll have two factors vaccine/placebo and <65 versus >= 65. For each of those we’ll have data on the number of people who got COVID and those who didn’t.
For our purposes we’re making some numbers up! They’ll be realistic and
possible given the limitations I laid out above (for example only 5 people
regardless of age got the true vaccine and got COVID). I’m
going to deliberately assign a disproportionate number of the positive
true vaccine cases to the older category a 4/1 split. Here’s a table,
of our little dataframe agg_moderna_data
.
Edit Nov 18th – I’m also aware that Pfizer has released even more data today that the vaccine is nearly equally effective for older people. But since I don’t have access to the exact numbers (if anyone has them send them along) we’ll make some up.
Factors of interest

COVID Infection

Total subjects



condition  age  didnot  got_covid  subjects 
placebo  Less than 65  8271  79  8350 
placebo  Older  4494  11  4505 
vaccinated  Less than 65  8293  1  8294 
vaccinated  Older  4497  4  4501 
Those familiar with frequentists methods in r
will recognize this as an
opportunity for glm
. For reasons I’ll explain later I would prefer to use the
aggregated data, although glm
is perfectly happy to process as a long
dataframe. So we’ll create a matrix of outcomes called outcomes
with
successes got_covid
and failures didnot
.
For binomial and quasibinomial families the response can be specified … as a twocolumn matrix with the columns giving the numbers of successes and failures
Voila the results using the native print
function as well as broom::tidy
.
With classic frequentist methodology we can now “reject” the null that the
\(\beta\) coefficients are 0. Both the main effects condition
and age
are
significant as well as the interaction age:condition
. Telling us that we must
look at the combination of age and vaccine status to understand our results.
outcomes < cbind(as.matrix(agg_moderna_data$got_covid, ncol = 1), as.matrix(agg_moderna_data$didnot, ncol = 1)) moderna_frequentist < glm(formula = outcomes ~ age + condition + age:condition, data = agg_moderna_data, family = binomial(link = "logit")) moderna_frequentist ## ## Call: glm(formula = outcomes ~ age + condition + age:condition, family = binomial(link = "logit"), ## data = agg_moderna_data) ## ## Coefficients: ## (Intercept) ageOlder ## 4.651 1.362 ## conditionvaccinated ageOlder:conditionvaccinated ## 4.372 3.360 ## ## Degrees of Freedom: 3 Total (i.e. Null); 0 Residual ## Null Deviance: 121.2 ## Residual Deviance: 5.18e13 AIC: 23.71 broom::tidy(moderna_frequentist) ## # A tibble: 4 x 5 ## term estimate std.error statistic p.value ## <chr> <dbl> <dbl> <dbl> <dbl> ## 1 (Intercept) 4.65 0.113 41.1 0 ## 2 ageOlder 1.36 0.322 4.22 0.0000240 ## 3 conditionvaccinated 4.37 1.01 4.34 0.0000140 ## 4 ageOlder:conditionvaccinated 3.36 1.16 2.89 0.00389
Notice that the \(\beta\) values are not especially informative because they are in link = “logit” format. We can infer direction. Older people, even though vaccinated still get COVID at higher rates. As we investigate bayesian methods we’ll convert back to our original scale. But let’s pause for a minute and acknowledge the limitations of the frequentist (NHST) method.
Things we can’t do (great reading here):
Claim that we are highly confident about our results because the p value is is very small? NO while you could construct confidence intervals around our \(\beta\) values you can’t make statements about probability, you either reject the null at some \(\alpha\) level or you don’t.
If the results were nonsignificant can you “accept” the null or say there is evidence for it. NO again that’s not permitted in the NHST framework.
Append this data to results from the study two months from now when the case count is higher? Another NO you can not legitimately keep adding data in a frequentist’s world.
A bayesian approach
brms
is great package that very much mirror’s the way glm
works. It
abstracts away many of the stumbling blocks that newcomers find
difficult about STAN
and bayesian modeling in general.
Let me back up a minute. The simplest way to run the bayesian analog if our data were in long format i.e. we had a dataframe with 25,650 rows one per subject, would be the following command.
brm(data = moderna_long_data, family = bernoulli(link = logit), y ~ age + condition + age:condition, iter = 12500, warmup = 500, chains = 4, cores = 4, control = list(adapt_delta = .99, max_treedepth = 12), seed = 9, file = "moderna_long")
We’re treating COVID19 outcomes as simple bernoulli events (a coin flip if you will). I’m providing the code so you can try it if you like. But I don’t recommend it. Expect a wait of 5 minutes or more!
Let’s go with a plan that works for the impatient (ME!). brm
allows us
to specify the outcomes aggregated. For each row of agg_moderna_data
it’s got_covid  trials(subjects)
or the number who became infected
by the number of subjects in that condition. Again our \(\beta\)’s are in
logit but we’ll correct that in a minute.
Notice file = "moderna_bayes_full"
which saves our results to a file on disk
named moderna_bayes.rds. That’s a blessing because we have the results saved and
can get them back in a future session. But be careful if you rerun the command
without deleting that file first it won’t rerun the analysis. We increased the
default number of chains and iterations and after burnin have 48,000 usable
samples.
moderna_bayes_full < brm(data = agg_moderna_data, family = binomial(link = logit), got_covid  trials(subjects) ~ age + condition + age:condition, iter = 12500, warmup = 500, chains = 4, cores = 4, seed = 9, file = "moderna_bayes_full") summary(moderna_bayes_full) ## Family: binomial ## Links: mu = logit ## Formula: got_covid  trials(subjects) ~ age + condition + age:condition ## Data: agg_moderna_data (Number of observations: 4) ## Samples: 4 chains, each with iter = 12500; warmup = 500; thin = 1; ## total postwarmup samples = 48000 ## ## PopulationLevel Effects: ## Estimate Est.Error l95% CI u95% CI Rhat Bulk_ESS ## Intercept 4.66 0.11 4.88 4.44 1.00 42732 ## ageOlder 1.39 0.33 2.08 0.79 1.00 21396 ## conditionvaccinated 4.80 1.21 7.73 2.98 1.00 11248 ## ageOlder:conditionvaccinated 3.72 1.35 1.45 6.80 1.00 12007 ## Tail_ESS ## Intercept 36581 ## ageOlder 21752 ## conditionvaccinated 10470 ## ageOlder:conditionvaccinated 11430 ## ## Samples were drawn using sampling(NUTS). For each parameter, Bulk_ESS ## and Tail_ESS are effective sample size measures, and Rhat is the potential ## scale reduction factor on split chains (at convergence, Rhat = 1). bayestestR::describe_posterior(moderna_bayes_full, ci = 0.95, test = c("p_direction"), centrality = "MAP") ## # Description of Posterior Distributions ## ## Parameter  MAP  95% CI  pd  Rhat  ESS ##  ## Intercept  4.651  [4.884, 4.437]  100.00%  1.000  42708.798 ## ageOlder  1.357  [2.050, 0.769]  100.00%  1.000  21032.449 ## conditionvaccinated  4.166  [7.274, 2.760]  100.00%  1.001  9684.929 ## ageOlder.conditionvaccinated  3.285  [ 1.308, 6.549]  99.96%  1.001  10879.160
Notice how similar our results are to glm
. We can also use
bayestestR::describe_posterior
directly on the brmsfit
object.
One of the very nice things about brms
is that it has plot
methods for
almost everything you might want to plot. A few examples below and even more
commented out.
mcmc_plot(moderna_bayes_full)
mcmc_plot(moderna_bayes_full, type = "areas")
mcmc_plot(moderna_bayes_full, type = "trace")
mcmc_plot(moderna_bayes_full, type = "hist")
mcmc_plot(moderna_bayes_full, type = "dens_overlay")
# mcmc_plot(moderna_bayes_full, type = "acf") # mcmc_plot(moderna_bayes_full, type = "neff") # mcmc_plot(moderna_bayes_full, type = "nuts_treedepth") # mcmc_plot(moderna_bayes_full, type = "scatter") # mcmc_plot(moderna_bayes_full, type = "rhat") # mcmc_plot(moderna_bayes_full, type = "dens")
Time to address the issue of getting our scale back to the original
instead of logit. brms
to the rescue by offering transformations
and more specifically transformations = "inv_logit_scaled"
. Now we can
see that \(\beta_0\) etc in their native scale.
mcmc_plot(moderna_bayes_full, transformations = "inv_logit_scaled")
Honestly though \(\beta\) coefficients are sometimes hard to explain to someone
not familiar with the regression framework. Let’s talk about conditional
effects. brms
provides a handy functional called conditional_effects
that
will plot them for us. The command conditional_effects(moderna_bayes_full)
is
enough to get us a decent output, but we can also wrap it in plot
and do
things like change the ggplot
theme. We can focus on just the interaction
effect with effects = "age:condition"
. This is important because interpreting
main effects when we have evidence of an interaction is dubious. We can even
pass additional ggplot
information to customize our plot.
# conditional_effects(moderna_bayes_full) plot(conditional_effects(moderna_bayes_full), theme = ggplot2::theme_minimal())
interaction_effect < plot(conditional_effects(moderna_bayes_full, effects = "age:condition", plot = FALSE), theme = ggplot2::theme_minimal())
interaction_effect$`age:condition` + ggplot2::ggtitle("Infection rate by condition") + ggplot2::ylab("Infected / Subjects per condition") + ggplot2::xlab("Age") + ggplot2::theme_minimal()
If this data were real it would give us pause since it highlights the fact that the vaccine appears to be much less effective for people over the age of 65.
If you don’t like the appearance of the plots from brms
or prefer
making use of bayestestR
as we did in earlier posts it is trivial
to extract the draws/chains and plot it.
# library(bayestestR) moderna_draws < tidybayes::tidy_draws(moderna_bayes_full) %>% select(b_Intercept:`b_ageOlder:conditionvaccinated`) %>% mutate_all(inv_logit_scaled) moderna_draws ## # A tibble: 48,000 x 4 ## b_Intercept b_ageOlder b_conditionvaccinated `b_ageOlder:conditionvaccinated` ## <dbl> <dbl> <dbl> <dbl> ## 1 0.00827 0.351 0.0160 0.948 ## 2 0.00954 0.164 0.0110 0.958 ## 3 0.00955 0.242 0.0202 0.962 ## 4 0.00876 0.240 0.0176 0.896 ## 5 0.00999 0.157 0.00496 0.981 ## 6 0.0129 0.171 0.00266 0.994 ## 7 0.00919 0.181 0.00442 0.993 ## 8 0.00971 0.244 0.0259 0.937 ## 9 0.00900 0.147 0.00749 0.982 ## 10 0.00893 0.172 0.00975 0.977 ## # … with 47,990 more rows plot(bayestestR::hdi(moderna_draws, ci = c(.89, .95)))
Priors and Bayes Factors
Bayesian analysis rests on the principle of modeling how the data inform our
prior beliefs about understanding. You’ll notice that no where above did I
specify any prior. That’s because brms
is kind enough to provide defaults.
From the documentation “Default priors are chosen to be non or very weakly
informative so that their influence on the results will be negligible and you
usually don’t have to worry about them. However, after getting more familiar
with Bayesian statistics, I recommend you to start thinking about reasonable
informative priors for your model parameters: Nearly always, there is at least
some prior information available that can be used to improve your inference.”
To be conservative but also to pave the way for computing some Bayes Factors
let’s consult the STAN
documentation
and some weakly informative priors. We can run get_prior
to get back
information about the defaults. We don’t really care about the overall
intercept \(\beta_0\) but let’s set the others to normal(0,3)
which
puts us between weak and very weak. We’ll store those changes in
bprior
, rerun the model with a new name and you can see that changing the
priors to something slightly more informative has little impact. The data
overwhelms the priors and our posteriors are about the same as they were.
get_prior(data = agg_moderna_data, family = binomial(link = logit), got_covid  trials(subjects) ~ age + condition + age:condition) ## prior class coef group resp dpar ## (flat) b ## (flat) b ageOlder ## (flat) b ageOlder:conditionvaccinated ## (flat) b conditionvaccinated ## student_t(3, 0, 2.5) Intercept ## nlpar bound source ## default ## (vectorized) ## (vectorized) ## (vectorized) ## default ### https://github.com/standev/stan/wiki/PriorChoiceRecommendations ### Generic weakly informative prior: normal(0, 1); bprior < c(prior(normal(0,3), class = b, coef = ageOlder), prior(normal(0,3), class = b, coef = conditionvaccinated), prior(normal(0,3), class = b, coef = ageOlder:conditionvaccinated)) bprior ## prior class coef group resp dpar nlpar bound ## normal(0, 3) b ageOlder ## normal(0, 3) b conditionvaccinated ## normal(0, 3) b ageOlder:conditionvaccinated ## source ## user ## user ## user full_model < brm(data = agg_moderna_data, family = binomial(link = logit), got_covid  trials(subjects) ~ age + condition + age:condition, prior = bprior, iter = 12500, warmup = 500, chains = 4, cores = 4, seed = 9, save_pars = save_pars(all = TRUE), file = "full_model") summary(full_model) ## Family: binomial ## Links: mu = logit ## Formula: got_covid  trials(subjects) ~ age + condition + age:condition ## Data: agg_moderna_data (Number of observations: 4) ## Samples: 4 chains, each with iter = 12500; warmup = 500; thin = 1; ## total postwarmup samples = 48000 ## ## PopulationLevel Effects: ## Estimate Est.Error l95% CI u95% CI Rhat Bulk_ESS ## Intercept 4.67 0.11 4.90 4.45 1.00 49307 ## ageOlder 1.33 0.32 1.99 0.74 1.00 29645 ## conditionvaccinated 3.97 0.76 5.67 2.69 1.00 15090 ## ageOlder:conditionvaccinated 2.76 0.96 0.96 4.75 1.00 16749 ## Tail_ESS ## Intercept 38807 ## ageOlder 26856 ## conditionvaccinated 15787 ## ageOlder:conditionvaccinated 18968 ## ## Samples were drawn using sampling(NUTS). For each parameter, Bulk_ESS ## and Tail_ESS are effective sample size measures, and Rhat is the potential ## scale reduction factor on split chains (at convergence, Rhat = 1).
Some bayesians like to work with a Bayes Factor. We can use
brms::bayes_factor
to inform us about the relative worthiness
of competing regression models. You’ll notice I named the first
set of results full_model
because it included age, condition and
the interaction between them age:condition. Let’s build three more
models:
 one without the interaction term called
no_interaction
,  one without the interaction or the vaccine called
no_vaccine
,  one without the interaction or age called
no_age
,
Then we can compare the full model against each of these nested models. (NB for the purists yes I actually tried some other priors that were less weak and yes the bayes factors are quite robust.)
bprior2 < c(prior(normal(0,3), class = b, coef = ageOlder), prior(normal(0,3), class = b, coef = conditionvaccinated)) bprior2 ## prior class coef group resp dpar nlpar bound source ## normal(0, 3) b ageOlder user ## normal(0, 3) b conditionvaccinated user no_interaction < brm(data = agg_moderna_data, family = binomial(link = logit), got_covid  trials(subjects) ~ age + condition, prior = bprior2, iter = 12500, warmup = 500, chains = 4, cores = 4, seed = 9, save_pars = save_pars(all = TRUE), file = "no_interaction") summary(no_interaction) ## Family: binomial ## Links: mu = logit ## Formula: got_covid  trials(subjects) ~ age + condition ## Data: agg_moderna_data (Number of observations: 4) ## Samples: 4 chains, each with iter = 12500; warmup = 500; thin = 1; ## total postwarmup samples = 48000 ## ## PopulationLevel Effects: ## Estimate Est.Error l95% CI u95% CI Rhat Bulk_ESS Tail_ESS ## Intercept 4.70 0.12 4.94 4.48 1.00 52909 38740 ## ageOlder 1.07 0.28 1.65 0.55 1.00 22774 21953 ## conditionvaccinated 2.87 0.45 3.83 2.08 1.00 18737 19745 ## ## Samples were drawn using sampling(NUTS). For each parameter, Bulk_ESS ## and Tail_ESS are effective sample size measures, and Rhat is the potential ## scale reduction factor on split chains (at convergence, Rhat = 1). bprior3 < c(prior(normal(0,3), class = b, coef = ageOlder)) bprior3 ## b_ageOlder ~ normal(0, 3) no_vaccine < brm(data = agg_moderna_data, family = binomial(link = logit), got_covid  trials(subjects) ~ age, prior = bprior3, iter = 12500, warmup = 500, chains = 4, cores = 4, seed = 9, save_pars = save_pars(all = TRUE), file = "no_vaccine") summary(no_vaccine) ## Family: binomial ## Links: mu = logit ## Formula: got_covid  trials(subjects) ~ age ## Data: agg_moderna_data (Number of observations: 4) ## Samples: 4 chains, each with iter = 12500; warmup = 500; thin = 1; ## total postwarmup samples = 48000 ## ## PopulationLevel Effects: ## Estimate Est.Error l95% CI u95% CI Rhat Bulk_ESS Tail_ESS ## Intercept 5.34 0.11 5.56 5.12 1.00 48533 33484 ## ageOlder 1.07 0.28 1.65 0.54 1.00 16100 17671 ## ## Samples were drawn using sampling(NUTS). For each parameter, Bulk_ESS ## and Tail_ESS are effective sample size measures, and Rhat is the potential ## scale reduction factor on split chains (at convergence, Rhat = 1). bprior4 < c(prior(normal(0,3), class = b, coef = conditionvaccinated)) bprior4 ## b_conditionvaccinated ~ normal(0, 3) no_age < brm(data = agg_moderna_data, family = binomial(link = logit), got_covid  trials(subjects) ~ condition, prior = bprior4, iter = 12500, warmup = 500, chains = 4, cores = 4, seed = 9, save_pars = save_pars(all = TRUE), file = "no_age") summary(no_age) ## Family: binomial ## Links: mu = logit ## Formula: got_covid  trials(subjects) ~ condition ## Data: agg_moderna_data (Number of observations: 4) ## Samples: 4 chains, each with iter = 12500; warmup = 500; thin = 1; ## total postwarmup samples = 48000 ## ## PopulationLevel Effects: ## Estimate Est.Error l95% CI u95% CI Rhat Bulk_ESS Tail_ESS ## Intercept 4.96 0.11 5.17 4.76 1.00 52371 33818 ## conditionvaccinated 2.88 0.45 3.85 2.07 1.00 10390 11347 ## ## Samples were drawn using sampling(NUTS). For each parameter, Bulk_ESS ## and Tail_ESS are effective sample size measures, and Rhat is the potential ## scale reduction factor on split chains (at convergence, Rhat = 1). bayes_factor(full_model, no_interaction, silent = TRUE) ## Estimated Bayes factor in favor of full_model over no_interaction: 31.06810 bayes_factor(full_model, no_vaccine, silent = TRUE) ## Estimated Bayes factor in favor of full_model over no_vaccine: 235611405759697158144.00000 bayes_factor(full_model, no_age, silent = TRUE) ## Estimated Bayes factor in favor of full_model over no_age: 17306.61482
We can now quantify how well our models “fit” the data compared to the nested models. The evidence for retaining the model with the interaction term included is about 30:1. The evidence for keeping age is about 17,000:1 and you don’t even want to think about dropping the vaccine condition.
I do not personally believe in any model that includes an interaction but not the main effect that accompanies it.
What have we learned?
Very little more about the actual data since I made some things up (full disclosure). But a lot about how to analyze it as it becomes available. As more data becomes available we’ll be able to do even more complex analyses with other factors like gender and race.
brms is a great modeling tool!
Done
I’m done with this topic for now. More fantastic news for humanity in the race for a vaccine.
Hope you enjoyed the post. Comments always welcomed. Especially please let me know if you actually use the tools and find them useful.
Keep counting the votes! Every last one of them!
Chuck
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