November 10, 2020

Covid vaccine

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So, there’s good news about the Pfizer vaccine.  Some context:

  1. What we have now is just a press release. However, the analysis and criteria were specified in advance, and we actually have that document, so it’s less fuzzy than it might be
  2. Data are still coming in, so the estimate of vaccine efficacy will change over time to some extent.  In particular, we wouldn’t have heard anything if the estimated efficacy wasn’t at least 63%, so the current estimate is likely a bit too high.  The current estimate is so far above the threshold of 63% that this bias shouldn’t be huge
  3. The trial focuses on preventing symptomatic infection. We haven’t heard anything about the impact on serious disease or on asymptomatic disease. The impact on serious disease is, oddly, less important, since the vaccine is good enough for herd immunity. However, if the vaccine (a bit implausibly) had no effect on serious disease and just made symptomatic infections asymptomatic, it wouldn’t be that much use.  Pfizer are collecting this information; it just wasn’t in the press release.
  4. The next important step isn’t peer-reviewed publication, it’s the FDA external advisory committee meeting. These are public and involve scientists and doctors external to the FDA who get to ask Pfizer questions and have them answered.  If the advisory committee is strongly in favour of emergency authorisation, I would expect Medsafe to reach the same conclusion.
  5. Duration of effect matters.  We cannot possibly know for another year whether protection lasts for a year (which would be plenty).  Very short duration of protection would still have some use for making travel safer and for ring-fencing small outbreaks, but it wouldn’t have much impact on the pandemic
  6.  New Zealand is in line for enough vaccine for  750,000 people, and Megan Woods says it could arrive early next year.  That’s not enough to have any noticeable impact on population spread, but it is enough to reduce transmission to border staff and healthcare workers. It might even allow some increase in the safe admission to NZ of temporary workers or students — the government needs to decide how to allocate the vaccine.  Expanding on this: a vaccine could be used to reduce the probability of an outbreak, to increase travel and help the economy, or to reduce the harm of an outbreak (eg vaccinating elderly people). These are all worthwhile and the detailed choice is a policy question.
  7. Mass vaccination won’t happen for a while.  Even if other candidate vaccines are effective (increasing the number of suppliers), mass vaccination in NZ is probably at least a year away
  8. If the current estimate holds up, the vaccine is effective enough that we might get reasonable herd immunity by vaccinating only people who actually want to be vaccinated, which would make life simpler.
  9. The Covid vaccines seem to have a higher rate of mild adverse effects than most vaccines we’re used to. It’s important not to deny these, and it would be useful if there were careful monitoring of adverse event rates in the first wave of NZ vaccine recipients by, eg, the NZ Pharmacovigilance Centre
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Thomas Lumley (@tslumley) is Professor of Biostatistics at the University of Auckland. His research interests include semiparametric models, survey sampling, statistical computing, foundations of statistics, and whatever methodological problems his medical collaborators come up with. He also blogs at Biased and Inefficient See all posts by Thomas Lumley »

Comments

  • avatar
    Nick Iversen

    The New York Times has a vaccine tracker at
    https://www.nytimes.com/interactive/2020/science/coronavirus-vaccine-tracker.html

    It says that 6 vaccines have been approved for early or limited release. I was wondering why we haven’t heard a lot more about these (we have just a couple of vague stories about a Russian and a Chinese vaccine)

    4 years ago

    • avatar
      Thomas Lumley

      There are four Chinese vaccines in addition to the two Russian ones. China has been clear that they’re doing emergency-use authorisation; Russia has been a lot less careful about details.

      4 years ago

  • avatar
    Kaye Gilhooley

    Re. point 6. Would another benefit be to allow unlimited entry to vaccinated visitors without the need for MIQ? Tourism industry would be back on then.

    4 years ago

    • avatar
      Thomas Lumley

      It’s not clear. First, there isn’t (at least currently) any biochemical way to check if someone has been vaccinated, so you’d be trusting some sort of paperwork that said they had. Second, we’d need to establish the duration of protection and the protection from asymptomatic as well as symptomatic infection. Third, if the prevalence is high enough where they are coming from, 90% protection might not be enough. In the medium term I think it would be more relevant to international students and specialist workers (film, yachting, agriculture) who aren’t going to travel widely within NZ. Eventually, though, yes.

      4 years ago

  • avatar
    Stuart Luppescu

    I had been hoping you would comment on the numbers in the study so far. At https://www.nytimes.com/live/2020/pfizer-covid-19-vaccine
    it says, “94 participants out of nearly 44,000 have gotten sick with Covid-19”. Are just 94 cases out of 44,000 people enough to make any kind of meaningful statement about the effectiveness of the vaccine?

    4 years ago

    • avatar
      Thomas Lumley

      Absolutely enough. In fact, the original plan with a first analysis at 32 would be enough to make a meaningful statement about effectiveness. The reason for running the trial longer before the first look is to get more data about safety — in particular, about potentially-serious rare side effects.

      We don’t have the data yet, but the consensus guess I’ve heard is probably 8 cases in the vaccine group and 86 in the control group. That’s a huge difference. There will be a lot of small-sample uncertainty about the precise effectiveness, but the lower bound is still pretty damn good.

      In fact, 94 observed cases is not especially small for a vaccine trial. There are a lot of smaller trials in, say, rotavirus vaccines.

      The large number of participants (44,000) were needed for two reasons. First, safety: you need to see thousands or tens of thousands of people not getting serious adverse reactions.

      Second, because you need to recruit a lot of people to reliably get 94 cases. Back in September, the US was seeing rates of about 1 case per 10,000 people per day, so you’d need a lot of people to get the planned cases in a reasonable time period. The second factor turned out to be less important that expected, since the population infection rate is higher than they had planned for.

      4 years ago