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The USA CDC director Dr. Robert Redfield was recently quoted in the media as saying:

"I might even go so far as to say that this face mask is more guaranteed to protect me against Covid than when I take a Covid vaccine, because the immunogenicity may be 70% and if I don't get an immune response, the vaccine is not going to protect me. This face mask will [...]"

What is the basis for that immunogenicity percentage projection?

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I don't think he's referring to anything specific. "May be" is not a definitive statement, it just seems he is suggesting that it is plausible that a vaccine could be less protective than regular mask wearing. "I might even go so far" couches it even more.

I think that's probably not a great statement to make, either, a better one might be to suggest that "because a vaccine is not effective in everyone that gets it, mask-wearing may continue to be important even with a vaccine", but not every quote in front of the media is ever ideal. Really, the rest of his statement:

if I don't get an immune response, the vaccine is not going to protect me. This face mask will

seems like the part to focus on.

For comparisons to influenza, this page talks about the annual flu vaccine efficiacy:

https://www.cdc.gov/flu/professionals/acip/immunogenicity.htm

A quote, emphasis mine:

IIV3 provided statistically significant protection against both the included B lineage (66%; 95%CI 58, 73) and the non-included B lineage (51%; 95%CI 36, 63) during the 2012– 13 season

Another paper,

Tricco, A. C., Chit, A., Soobiah, C., Hallett, D., Meier, G., Chen, M. H., ... & Loeb, M. (2013). Comparing influenza vaccine efficacy against mismatched and matched strains: a systematic review and meta-analysis. BMC medicine, 11(1), 153.

is a meta-analysis of flu vaccine efficacy in children. They write, quote, with emphasis mine:

The live-attenuated influenza vaccine (LAIV) showed significant protection against mismatched (six RCTs, VE 54%, 95% confidence interval (CI) 28% to 71%) and matched (seven RCTs, VE 83%, 95% CI 75% to 88%) influenza strains among children aged 6 to 36 months

VE was calculated using the following formula: (1 - relative risk × 100%)

So it seems that 70% would be in the ballpark of the annual flu vaccine against the included strain. Obviously the 2019 coronavirus is not influenza, but we have no solid long-term efficacy data yet, and I think it's reasonable to expect that even a very useful vaccine (on par with the annual flu vaccine given to millions) would not approach 100% efficacy. Even with a vaccine, it will probably be important to continue basic public health protocols intended to limit virus spread, especially before many people are immunized.

There's some mixing here between "immune response" and "efficiacy" but I think it would be fair to say that if someone gets sick, then whatever measured immune response they may have had was not effective, so we might as well consider it not an immune response for this purpose. Rewriting a paraphrased version of the quote, I would say:

"If the Covid vaccine provides similar protection to the annual influenza vaccine, the immunogenicity could be around 70%. If I don't get an immune response, the vaccine is not going to protect me but a face mask will still provide some protection."

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From a recent review of "Warp Speed" vaccine candidates

General experience, combined with emerging data, suggests that the most rapidly produced vaccines (i.e., nucleic acids and virus vectors) may also be the least capable of eliciting high titers of antibodies and NAbs to the S-protein.

So, it's probably a safe bet to limit discussion to those for now... The review also says that

it is unknown whether the results obtained in small animals will be matched in humans. The immunogenicity of mRNA and DNA vaccines is generally far stronger in small animals than in macaques, and more so in humans.

And for the latter it cites a paper on a MERS DNA vaccine, which interestingly happens to mention something like those ~70% numbers....

T-cell responses were detected in 47 (71%) of 66 participants after two vaccinations and in 44 (76%) of 58 participants after three vaccinations. [...] At week 60, vaccine-induced humoral and cellular responses were detected in 51 (77%) of 66 participants and 42 (64%) of 66, respectively.

There's also been a bit of a debate as to what measure(s) of immunity are most relevant. Most of the (short-term) Covid-19 vaccine data published so far, itself on the order of one-month, seems to have been mainly anti-bodies (e.g. a recent study on one of the mRNA vaccines found average levels exceeding convalescence), but some researchers disagree this is the most relevant measure, pointing to T-cells response as more relevant. Actually the T-cells measure was reported for that vaccine candidate too, in a separate publication/preprint; it was on the order of 80% response [29/36 participants] for CD8+ T cells. And that seems to be as good as it gets in terms of Covid-19 phase-1 published data.

I guess there's some "fudge factor" to consider here given that phase I trials tend to select healthy volunteers in the 18-55 age range etc.

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Without questioning the CDC director specifically we are left to speculate why he said 70% as contrasted with 60% or 80%. However, the protocol released by Pfizer may give some clue. The clinical trial for their candidate vaccine will last approximately two years, however they will be conducting Interim Analysis for Vaccine Efficacy at several points during that two year period.

Pfizer will consider the vaccine an interim success, if after 32 persons contract Covid19, only 6 of those people were injected with the trial vaccine. That calculates to an efficacy of 77%.(see page 102 of protocol)

Subsequent interim analysis the last being after 164 persons contract Covid-19. If, of the 164 persons only 53 were injected with the trial vaccine, that would also be deemed a success. That calculates to an efficacy of 52%. (same page)

So from the Pfizer protocol we see efficacy targets for success of 52% or greater.

However, Efficacy (the extent to which a vaccine provides a beneficial result) is not synonymous with Immunogenicity (the ability of a vaccine to provoke an immune response creating antibodies). Having antibodies (naturally or provoked) against COVID19 is clearly better than having no antibodies, but there is no assurance that any of the trial vaccines will produce antibodies in every single person (100% immunogenicity). As a practical matter the immunogenicity will likely be less than 100%, but 70% would be considered a great success, particularity if the antibodies are constantly being produced and renewed. For those who do not develop antibodies, or that their bodies do not continue to produce "fresh" antibodies they will have to rely on other protective means - such as masks.

The Pfizer protocol is silent on the immunogenicity criteria for success.

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