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Respective Per Capita Rates, February 25

By February 27, 2022Commentary

An interesting look at per capita rates of infections for breakthroughs, reinfections and the unvaxed with no detected prior infections.  As usual, the big issue is the overlap, which certainly exists, between the prior infected and vaxed populations.  DOH can figure this out but doesn’t reveal it.  As Dave notes, we don’t know respective rates of undetected infections in each group.  During Omicron, possibly due to waning immune response, possible due to immune evasion by Omicron, the rates have converged.  This effect noticed in the Minnesota data has also been reported in various pieces of research.  Of  note is the sharp convergence in vaxed and unvaxed rates of infection.

Dave’s notes:

  • It has been quite a few weeks since we last published the chart showing relative rates of cases per 100k for Covid reinfections, breakthroughs, and first cases among the unvaccinated, here:  At that time all of our data, because of lags in getting breakthrough data from the Minnesota Department of Health (MDH), was from the time period just before the start of Omicron surge in cases.
  • Late on the afternoon of 2/25/2022 we received a refresh of the breakthrough cases by date of specimen collection, in response to a Data Practices Act request we had filed with MDH. This was the fourth set of data on breakthrough cases we have been provided.
  • One important limitation of this chart, among several, is that MDH does not disclose if any of the reinfection cases occurred among vaccinated people. We display all reinfections as one category. In addition, we know that a reinfection in a unvaccinated person is counted as new case as well as a reinfection. Is a reinfection in a vaccinated person counted both as a reinfection and as a breakthrough case? We have filed a Data Practices Act request in an attempt to understand how reinfections among the vaccinated are accounted for.
  • Since 11/2/2021 MDH has published Covid reinfection data on the Minnesota Situation Update for COVID-19 web page:  Reinfections are defined as a person testing positive for Covid more than 90 days after a prior positive test.
  • “Confirmed” reinfections are defined as a positive PCR test following a positive antigen test.  Probable is defined as a negative PCR test following a positive antigen test. It is implied, similar to Cases, that a PCR positive test is considered “Confirmed” and a positive antigen test is considered “Probable”.  For purposes of this analysis all Confirmed and Probable reinfections will be included.
  • The weekly Covid reinfection rate displayed on the chart is calculated as the number of weekly reinfections (confirmed and probable) divided by the cumulative Total Positive People 90 days prior to the start of the week being considered, and the result multiplied by 100k to yield the reinfection rate per 100k per week. This is the red curve on the chart. We calculate Total Positive People by subtracting the cumulative reinfections from the cumulative positive cases. We do not use the cumulative Total Positive People provided by MDH because of inconsistencies in their total at different times in the pandemic, and because they only started providing this total on 11/1/2021.
  • The Fully Vaccinated Age Adjusted Case Rate Per Week (green curve) and Not Fully Vaccinated Age Adjusted Case Rate Per 100k (blue curve) are taken directly without modification from the Vaccine Breakthrough Report from the data file for Cases, Hospitalizations, and Deaths Over Time graphic (vbtaarates.xlsx linked on this page).
  • Data is plotted starting 5/2/2021, the earliest date we have breakthrough case rates for the vaccinated and unvaccinated from MDH. Reinfection data goes back to 6/28/2020, but the earlier data is more variable possibly due to the smaller population of previously infected people. In the future we may extend this chart further back in time.
  • We have no knowledge of what age adjustments DOH performs in order to calculate the Age Adjusted Rates, but we have submitted a formal Data Practices Act Request to MDH for the underlying methodology and detailed calculation.
  • It is likely that there are people who may have contracted a mild enough Covid infection that they did not seek medical attention or get tested, causing an undercount of the true number of breakthrough infections, non-breakthrough infections, and reinfections.
  • We do not know how thorough and comprehensive MDH’s process is in identifying reinfections or breakthrough infections.
  • The main purpose in generating this chart is to try to compare the relative infection rates for previously infected people to vaccinated people and to unvaccinated people. Throughout the end of 2021 the ratio of vaccinated case rate to reinfection case rate was typically between 3:1 to 4:1, while the ratio of unvaccinated case rate to reinfection case rate was between 10:1 to 14:1. However, during the Omicron surge these ratios changed, as the reinfection case rate rose to nearly equal the vaccinated case rate. At the peak of the Omicron wave the ratio of the vaccinated case rate to the reinfection case rate was only 1.1:1, while the ratio of the unvaccinated case rate to the reinfection case rate was 1.9:1. A similar comparison can be made between the vaccinated and unvaccinated. In the Fall of 2021 the unvaccinated were between 3 and 4 times as likely as the vaccinated to test positive. At the peak of the Omicron surge the unvaccinated were only 1.4 times as likely to test positive as the vaccinated.

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