study by the U.S. CDC (a Federal Government health agency) has found that natural immunity is superior to vaccine immunity alone, and that being vaccinated on top makes little difference, confirming the findings of several studies in other countries.

The study looked at Covid infection and hospitalisation rates in California and New York during the Delta period, June to November 2021. The chart below shows the hazard rates over time for the four cohorts (no vaccine and no prior infection; no vaccine and prior infection; vaccine and no prior infection; vaccine and prior infection) for hospital admissions, adjusted for age – focus especially on the relative heights of the three dashed lines near the bottom.

The authors write: “These results demonstrate that vaccination protects against COVID-19 and related hospitalisation, and that surviving a previous infection protects against a reinfection and related hospitalisation.” Note that the unvaccinated group here is split between those who are and are not previously infected, so is not directly comparable to the UKHSA data which does not make this distinction.

The authors note that, while prior to July the vaccinated were better off than the previously infected, come the summer and the arrival of Delta, prior infection was stronger. 

Similar to the early period of this study, two previous U.S. studies found more protection from vaccination than from previous infection during periods before Delta predominance. As was observed in the present study after July, recent international studies have also demonstrated increased protection in persons with previous infection, with or without vaccination, relative to vaccination alone.

In a nod to the superior effect on the immune system of exposure to the virus, they comment : “This might be due to differential stimulation of the immune response by either exposure type.”

The vaccines waned in efficacy over time and against Delta, the authors note: “Importantly, infection-derived protection was greater after the highly transmissible Delta variant became predominant, coinciding with early declining of vaccine-induced immunity in many persons.”

In contrast, protection from natural immunity didn’t appear to wane: “Whereas French and Israeli population-based studies noted waning protection from previous infection, this was not apparent in the results from this or other large U.K. and U.S. studies.”

The study findings were adjusted for age and controlled for prior infection status, but not for any other potential confounders, such as “behavioral or geographic differences in exposure risk”, differences in testing behaviour, and “uncertainty in the population size of the unvaccinated group without a previous COVID-19 diagnosis”. Unlike the UKHSA, which repeatedly states in its reports of similar data that such confounders are unknown and render the data unusable for estimating vaccine effectiveness (an equivalent quantity to hazard rate), the CDC authors just state: “Different trajectories between vaccinated persons with and without a previous COVID-19 diagnosis, and similar findings for cases and hospitalisations, suggest that these biases were minimal.” 

Which prompts the question: why is UKHSA making such a fuss about how unusable its data supposedly is for estimating vaccine effectiveness? Why can’t we have a similar report from the UKHSA, using similar methods? Note that UKHSA data is already effectively controlled for age because it is divided into age bands, and the CDC is content that other biases are so minimal as not to worry about them. This makes the UKHSA’s position even more untenable, and further pressure should be placed on it to produce a similar study using its data.

This study from a U.S. federal agency is particularly welcome as it means all the people claiming that vaccines are better than natural immunity can be pointed to a source they will presumably regard as authoritative to set them straight.

Of course, this is all pre-Omicron, and Omicron evades immunity much more than Delta – though vaccine immunity more than natural immunity, meaning natural immunity is still stronger. One study suggests natural immunity is better against Omicron than three vaccine doses, and more resilient.

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