For some, now may be the time to get your third Covid shot


  • It seems likely there is a significant benefit to a booster shot in preventing symptomatic infection in the face of the Delta variant for many people (Bullets 1-3).

  • There is significant indication that a third dose of an mRNA vaccine has a good safety profile (Bullets 2-4).

  • Delta may have an a greater impact in the US than you realize (Bullet 5), and

  • Therefore, for some people now may be the right time to get their third Covid mRNA shot (Bullet 6) although there are many reasons to potentially wait (Bullet 7).


  1. Two days ago, the Israeli government announced that its observed vaccine efficiency (Pfizer) as of June 6th was down to 64% against symptomatic infection, while protection against severe disease and hospitalization has declined more mildly to 93%. This is seemingly almost entirely attributed to the Delta variant.

  2. Today, Pfizer put out a statement which included the following:

    Pfizer and BioNTech have seen encouraging data in the ongoing booster trial of a third dose of the current BNT162b2 vaccine. Initial data from the study demonstrate that a booster dose given 6 months after the second dose has a consistent tolerability profile while eliciting high neutralization titers against the wild type and the Beta variant, which are 5 to 10 times higher than after two primary doses. The companies expect to publish more definitive data soon as well as in a peer-reviewed journal and plan to submit the data to the FDA, EMA and other regulatory authorities in the coming weeks. In addition, data from a recent Nature paper demonstrate that immune sera obtained shortly after dose 2 of the primary two dose series of BNT162b2 have strong neutralization titers against the Delta variant (B.1.617.2 lineage) in laboratory tests. The companies anticipate that a third dose will boost those antibody titers even higher, similar to how the third dose performs for the Beta variant (B.1.351). While Pfizer and BioNTech believe a third dose of BNT162b2 has the potential to preserve the highest levels of protective efficacy against all currently known variants including Delta, the companies are remaining vigilant and are developing an updated version of the Pfizer-BioNTech COVID-19 vaccine that targets the full spike protein of the Delta variant.

CNN says they plan to submit to the CDC for emergency approval to administer booster doses next month.[1]

  1. Moderna previously announced that in their trial they also observed indication of significant benefit to a third dose booster (of half the size of the initial Moderna doses)[2] and found it to have a similar safety profile to the second shot.

  2. In addition to the clinical trials, Turkey, the UAE, NYC, and some others have given people third doses in some circumstances /​ categories. The immunocompromised are particularly likely to benefit, as one would expect.

  3. The Delta variant may have more of an impact than you expect. Polymarket currently gives a ~⅔ probability to the US again having more than 100,000 cases in a day before the end of the year. This could come from a new variant or other novel situation, but I expect most of this probability for most bettors is with an eye toward the Delta variant. Cases in the UK have increased 10-20x since their post-vaccination trough, and it has/​had similar vaccination rates to the US (although less immunity from having contracted Covid, and a greater proportion with only one dose or a less effective vaccine). The UK’s current number of new cases is equivalent to ~160k new daily cases in the US when adjusted for population.

  4. There are various reasons why getting vaccinated now may be particularly good for you

    1. Symptomatic infection may be bad to you, especially if you or someone you interact closely with is particularly vulnerable.

    2. If you received your vaccine early (e.g. 2020-February 2021) your protection is more likely to have weakened.

    3. Your peak exposure may be soon if the Delta variant has a similar effect in the US as it did in the UK. You’d also need to plan for the 1-2 week delay between shot and effect.

    4. When a booster shot likely does get approved by the CDC in ~1-3 months, there may be a run on supply preventing getting a vaccine booster for some time.

  5. There are also various reasons to not pursue a booster vaccine now

    1. You may care primarily about protection against severe disease, which seems to largely be maintained in the face of the Delta variant.

      1. You may be in good health /​ young, in which case you have additional reason to believe you will be fine even if you contract Covid.

    2. If less time has passed (under 4-5 months?) since you were vaccinated, then your potential benefit is likely less than some bullets above probably suggested it was for those who have had more time pass.

    3. Some, such as the CDC and FDA, don’t think a booster is currently warranted.

    4. It’s unclear to me if it’s legal to get a third dose in the US prior to that being approved. Regardless, it may be hard to procure without misrepresentation of your previous vaccinations.

    5. More data on safety, optimal timing, etc. will be forthcoming and may help you make a better decision.

    6. Boosters may be available this fall that specifically target newer variants, and perhaps getting a booster now will in some way make procurement of those worse (worse side effects, mandatory wait time since last vaccine, etc).

    7. Getting an additional dose of vaccine may ‘take it away’ from somebody else who needs it more, due to global supply limitations.

    8. You may believe the Delta variant is unlikely to lead to a large wave in your area, e.g. because of local vaccination rates or the ‘control system’.

For some, now may be the time to get your third Covid shot.

  1. Summary article of these developments. ↩︎

  2. For this reason, if pursuing a third booster shot soon, I’d lean Pfizer (smaller dose size) over Moderna. ↩︎