The New CDC Study on Natural Immunity

Disclaimer: I am not a medical doctor and this is not medical advice. My goal is to empower you with information. I will not take a position on whether you should or should not get vaccinated. Please make this decision yourself, consulting sources you trust, including a caring health care professional.

In my October 23 review of the immunity to infection provided by COVID vaccines versus previous COVID infection, I found nine studies that directly compared infection risk between unvaccinated previously infected people and fully vaccinated but never-infected people. The conclusion was that, across these studies, natural immunity is equal to or superior to full vaccination, and in some contexts it is wildly superior.

To reduce wordiness, in most cases below I will simply refer to fully vaccinated, never-infected people as “vaccinated,” and unvaccinated people with previous infection as “naturally immune.”

Within a week, the “CDC” came out with a new study claiming that vaccination provides five times better protection than natural immunity, concluding that “All eligible persons should be vaccinated against COVID-19 as soon as possible, including unvaccinated persons previously infected with SARS-CoV-2.”

I put “CDC” in quotation marks because although this was published in the CDC's journal, Morbidity and Mortality Weekly Report (MMWR), and several of the authors are affiliated with the CDC's COVID-19 Response Team, many of the authors are affiliated with pharmaceutical companies such as Pfizer, Astrazeneca, Merck, and GlaxoSmithKline.

This is best described as a collaboration between authors from the CDC, major pharmaceutical and health care companies, hospitals, and medical schools published in the CDC's journal.

MMWR is not peer reviewed, but the editors review each article to ensure it “comports with CDC policy,” so it has undergone more review than a preprint* has, but hasn't been reviewed for scientific rigor the way peer-reviewed papers in regular science journals have.

This New Study Doesn't Fit the Criteria for My Earlier Review

This study cannot change the conclusions of my earlier review because it simply doesn't fit the criteria I used when collecting the nine studies on infection risk: it does not compare the risk of getting COVID-19 among those who were vaccinated versus naturally immune from representative samples of those respective populations.

Rather, it starts with those who have been hospitalized for COVID-like illness and makes that comparison within those who had been hospitalized.

This is a rather bizarre design because, presumably, our interest in the relative risk of getting COVID-19 is that we want fewer people to be hospitalized with respiratory failure or other severe symptoms. But if everyone in the study is hospitalized for those symptoms, how much does it matter whether their PCR test was positive or negative?

This study does suggest that vaccination makes people more likely to be hospitalized with a negative PCR test rather than a positive one, but it doesn't support vaccination making people any less likely to wind up in the hospital.

Although it can't change the conclusions of my earlier review, it is still quite interesting to look at. What, exactly, does this paper show?

Who Was Studied

This study was done on adults who were hospitalized in 2021 with respiratory failure, pneumonia, cough. fever, difficulty breathing, vomiting, or diarrhea, who had at least one PCR or rapid antigen test prior to this hospitalization, and had at least one PCR test in the two weeks leading up to or the 3 days just after admission. The data came from 187 hospitals across nine states. Partially vaccinated people, those who recieved the J&J vaccine instead of an mRNA vaccine, and those whose vaccination or previous infection occurred less than 90 days earlier were excluded.

So, this study is asking the following question:

Among those hospitalized with COVID-like symptoms, which event from more than 90 days in the past is more effective at reducing the probability of a positive PCR test, natural infections or fully completed mRNA vaccinations?

Between January 1 and September 2, 2021, 201,269 people were hospitalized with COVID-like illness. Two thirds of these people were tested for COVID, and just under eight percent of them also had a COVID test from the earlier period. From among over 200,000 hospitalized people, 7,348 people had two tests from the appropriate time periods and fit into the categories of vaccinated or naturally immune.

Six Times More Fully Vaccinated Hospitalized Patients?

6,328 vaccinated and 1,020 naturally immune people were hospitalized for COVID-like illness.

The study wasn't designed to look at the relationship between vaccination and COVID-like illness, so they never report the vaccination rate in the large group of over 200,000 people. If it looks anything like the rate in this subgroup, we should be very alarmed.

Although we don't have the vaccination rate among the populations from which these hospitalized individuals came and thus have no denominator, the fact that there were over six times as many vaccinated people hospitalized for COVID-like illness as naturally immune people should make us wonder whether the vaccines might increase the risk of being hospitalized for COVID-like illness. At a minimum, it suggests that when we see data about people hospitalized with COVID, it may be missing a larger fraction of the hospitalized population with PCR-negative COVID-like illness.

This concern is reminiscent of the point the BMJ's Peter Doshi had made when he pointed out that the FDA's review of Pfizer's data for the EUA given in December 2020 showed that PCR-positive COVID was only 5% of all COVID-like illness, and the vaccine appeared to have no effect on total COVID-like illness. In that two-month followup data, only two cases of suspected COVID were serious and both were in the vaccine group.

I imagine the six-month followup has more serious cases of COVID-like illness, but to my knowledge, there is no FDA briefing available for that data because the advisory committee was never convened to vote on the full approval. None of the publications from the clinical trials report the amount of COVID-like illness or how many people were hospitalized from it. So the clinical trial data that hasn't been released may or may not support the surface appearance from this most recent paper that makes vaccines appear to promote hospitalization for COVID-like illness.

We cannot conclude anything for sure from this, but it looks suspicious.

The Main Results of the Study

Among the 6,328 vaccinated patients, 5.1% tested positive. Among the 1,020 naturally immune patients, 8.7% tested positive.

One way to look at this is that naturally immune patients were 71% more likely to test positive than vaccinated patients.

Another way to look at it is that 94.4% of COVID-like illness was PCR-negative. This is very similar to what Doshi dug up in the FDA's review of the 2-month Pfizer data, only in this case everyone was hospitalized.

And remember, they are all hospitalized from respiratory failure, pneumonia, cough, fever, difficulty breathing, vomiting, or diarrhea.

The biggest news from this should be that hospitalization with these symptoms, if the PCR test is worth its salt, generally isn't driven by COVID.

When they statistically adjusted for age, geographic region, calendar time, and local virus circulation, naturally immune patients had 5.5 greater odds of testing positive than vaccinated patients.

This was overwhelmingly driven by people over age 65. When they were separated from their younger counterparts, the adjusted odds for natural immunity vs vaccination in the over-65 patients shot up to 19.6, and in the younger crowd it was only 2.6.

Although they captured vaccinations and natural infections from the same broad period of time, the vaccinations were still more recent. 75% were in July or August and 96% were at least as recent as June. By contrast, for the natural infections, 69% were in May or earlier and 87% were at least as old as June. This trend is also reflected in the number of days between the first immunity event and the hospitalization, with more than twice as many previous infections being 150-179 days old, and 47% more vaccinations being 90-119 days old. Nevertheless, they did control for this in one model, and although it seemed to reduce the odds down from 5.5 to 4.0, the difference was not statistically significant.

Moderna (7.3) was more effective than Pfizer (5.1).

The association was, surprisingly to me, somewhat stronger among the delta-dominant period (7.55) than the before-delta period (6.11) but that also wasn't significant.

So What Do We Conclude?

This study suggests that vaccination is more effective than natural immunity at reducing the likelihood of a positive PCR test among people who are all hospitalized for the same reason, and that this is driven mainly by an effect in the over-65 crowd.

It does not tell us anything about the likelihood of testing positive in general, or the likelihood of being hospitalized for these symptoms.

In fact, it raises the question of whether the vaccines may be increasing the likelihood of winding up in the hospital for respiratory failure, pneumonia, cough, fever, difficulty breathing, vomiting, or diarrhea. However, it was not designed to ask that question, so we cannot form any clear conclusions from it.

Because this study has no non-hospitalized people in its denominator, it doesn't even tell us the risk of winding up as a PCR-positive hospitalized case. If they want us to assume the denominators would be proportional, such that in the general population 70% more naturally immune people than fully vaccinated people would wind up hospitalized with COVID, then we should also assume the denominators are proportional for all COVID-like illness and assume that vaccinated people are six times more likely to wind up hospitalized with it.

This paper does not contradict anything from the conlusions of the nine studies I reviewed earlier. These studies continue to show that natural immunity is equal to or superior to full vaccination at protecting against infection risk, and in some contexts natural immunity is wildly superior. These studies show the same thing about the risk of serious illness and hospitalization.

My conclusion from this is in complete contrast to the authors' —  “All eligible persons should be vaccinated against COVID-19 as soon as possible, including unvaccinated persons previously infected with SARS-CoV-2” — which I find totally incomprehensible outside of the conflict of interest statement showing many of the authors are affiliated with Big Pharma.

The data shows that naturally immune people might benefit from “hybrid immunity” but certainly don't need any vaccination to have the same or better immunity as the fully vaccinated, never-infected.

Video With Q&A

Slide Deck

Here is a slide deck I made for this analysis.


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*  The term “preprint” is often used in these articles. Preprints are studies destined for peer-reviewed journals that have yet to be peer-reviewed. Because COVID-19 is such a rapidly evolving disease and peer-review takes so long, most of the information circulating about the disease comes from preprints.

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  1. Regarding the PCR test, didn’t “they” (CDC or other powers that be) decide that a lower threshold of 28 cycles would be used for determining Covid in vaccinated persons and 35 cycles for non-vaccinated. If that’s the case then the likelihood of false positives in non-vaccinated persons could account for the differences in this study.

  2. Statistics, statistics everywhere but no one ever shows from where the numbers are derived or what assumptions are used in the analysis to reach a conclusion! Regardless of subject matter, we are supposed to believe everything they tell us because of their math. Just trust the source, right? I’ve been doing it too long for that. “ Figures may not lie, but liars figure!”😁

  3. This study reeks of cherry picked data. Reading between the lines, it suggests that J&J is garbage, and natural immunity is probably superior in the less than 90 days from immunity generating event data.

  4. The movie Dark Waters shows conspiracy between big business and government. These shenanigans are nothing new! I just can’t believe the general populace fall for it every time. I’ve just had the worst respiratory type virus in my life, coughing up phlegm for 6 weeks. PCR neg and double Moderna jabbed against my better judgement… In an experiment of 1 i.e. me I’m sure the vaccine made things worse.. what a world. Funny thing is I’d never take a statin so I don’t know why I took this vaccine..

  5. From the study – ‘Previous infection was ascertained based on SARS-CoV-2 testing from rapid antigen tests or molecular assays (e.g., real-time reverse transcription–polymerase chain reaction)’.
    They didn’t test for antibodies so they don’t know how many of those people who tested positive previously had immunity. How many only had a tiny dose of virus that never took hold or was eliminated before they could develop antibodies?
    How can they claim ‘vaccine-induced immunity was more protective than infection-induced immunity’ when they don’t know who had infection-induced immunity?

  6. Why was the naturally immune group (~1k) so much smaller than the vaccinated group (~6.3k)? Could it perhaps be that the authors could not find 6.3k naturally immune people in the hospital? If so, wouldn’t that suggest that natural immunity is much better at keeping people out of the hospital, compared to the vaccine?

  7. Biased science is bad science.
    ‘Science” driven by profit is bad science.
    “Science” driven by an agenda to control people against their best interests is certainly the worst kind of bad science.

  8. I don’t think I’ll ever trust public health officials again. It was already bad before this outbreak but now it’s blatant. Thanks Chris!

    1. I data crunched this study. You might want to separate out the data (the table is in the study) for pre-delta vs delta. The authors leave out the fact that in a post delta environment, natural immunity wins based on their own data.

  9. Hi Chris

    It seems common sense to me that prior infection will give greater protection to ANY infection than a vaccine since the body’s response to full-blown infection is massive compared to the response elicited by vaccine. It also seems common sense that almost any vaccine you can point at will give at least some protection compared with no vaccine at all. And since we really don’t know how sick we might get when infected, vaccination makes sense – a prick in the arm seems a small price to pay.

    If I am missing something, show me where it is in your article.

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