Friday, January 21, 2022

MMWR: COVID-19 Cases & Hospitalizations by COVID-19 Vaccination Status and Previous COVID-19 Diagnosis


Two days ago the CDC's MMWR published a new report that found - over a specific period of time, before the emergence of Omicron - that people who had recovered from a previous bout of COVID were better protected against reinfection with the Delta strain, than those who were only vaccinated. 

Many who oppose the vaccine, or who believe a prior infection should exempt one from getting the vaccine, took to social media to declare victory. But as usual, there is more to unpack here than  can be posted in a twitter taunt. 

The highest rate of hospitalization were among persons who were unvaccinated (without previous infection hx) Prior to the arrival of Delta, vaccinated individuals were better protected (against hospitalization) than those unvaccinated but with previous infection

The change came after Delta become dominant. Suddenly prior infection provided better protection than the vaccine alone.

By the week beginning October 3, compared with COVID-19 cases rates among unvaccinated persons without a previous COVID-19 diagnosis, case rates among vaccinated persons without a previous COVID-19 diagnosis were 6.2-fold (California) and 4.5-fold (New York) lower; rates were substantially lower among both groups with previous COVID-19 diagnoses, including 29.0-fold (California) and 14.7-fold lower (New York) among unvaccinated persons with a previous diagnosis, and 32.5-fold (California) and 19.8-fold lower (New York) among vaccinated persons with a previous diagnosis of COVID-19.

The combination of prior infection AND vaccination provided the best level of protection, but - as we've discussed many times - the COVID vaccine alone was struggling against Delta.  

And of course, since this study, Delta has been supplanted by Omicron, which appears to have a much higher reinfection and breakthrough rate than Delta. 

First stop, a link and some excerpts from the MMWR report (which you'll want to read in its entirety), followed by a statement by the CDC. 

COVID-19 Cases and Hospitalizations by COVID-19 Vaccination Status and Previous COVID-19 Diagnosis — California and New York, May–November 2021

Early Release / January 19, 2022 / 71

Tomás M. León, PhD1; Vajeera Dorabawila, PhD2; Lauren Nelson, MPH1; Emily Lutterloh, MD2,3; Ursula E. Bauer, PhD2; Bryon Backenson, MPH2,3; Mary T. Bassett, MD2; Hannah Henry, MPH1; Brooke Bregman, MPH1; Claire M. Midgley, PhD4; Jennifer F. Myers, MPH1; Ian D. Plumb, MBBS4; Heather E. Reese, PhD4; Rui Zhao, MPH1; Melissa Briggs-Hagen, MD4; Dina Hoefer, PhD2; James P. Watt, MD1; Benjamin J. Silk, PhD4; Seema Jain, MD1; Eli S. Rosenberg, PhD2,3 (View author affiliations)View suggested citation


What is already known about this topic?

Data are limited regarding the risks for SARS-CoV-2 infection and hospitalization after COVID-19 vaccination and previous infection.

What is added by this report?

During May–November 2021, case and hospitalization rates were highest among persons who were unvaccinated without a previous diagnosis. Before Delta became the predominant variant in June, case rates were higher among persons who survived a previous infection than persons who were vaccinated alone. By early October, persons who survived a previous infection had lower case rates than persons who were vaccinated alone.

What are the implications for public health practice?

Although the epidemiology of COVID-19 might change as new variants emerge, vaccination remains the safest strategy for averting future SARS-CoV-2 infections, hospitalizations, long-term sequelae, and death. Primary vaccination, additional doses, and booster doses are recommended for all eligible persons. Additional future recommendations for vaccine doses might be warranted as the virus and immunity levels change.

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Anticipating that this MMWR report might be misconstrued, the CDC published the following statement on Wednesday. 

CDC Statement on MMWR: COVID-19 Cases and Hospitalizations by COVID-19 Vaccination Status and Previous COVID-19 Diagnosis — California and New York, May–November 2021
Media Statement

Embargoed Until: Wednesday, January 19, 2022, 1:00 p.m. ET
Contact: Media Relations
(404) 639-3286

Today’s MMWR study finds that during the Delta wave, both COVID-19 vaccination and surviving a prior infection provided protection against infection and hospitalization from COVID-19. Scientists reviewed data from New York and California to determine the level of protection offered by COVID-19 vaccines, previous infection, and both. Between May and November 2021, people who were unvaccinated and did not have a prior COVID-19 infection remained at the highest risk of infection and hospitalization, while those who were previously infected, both with or without prior vaccination, had the greatest protection.

Viruses are constantly changing, including the virus that causes COVID-19. These changes occur over time and can lead to the emergence of new variants that have new characteristics, including ones that impact the level of immunity vaccination and/or prior infection can provide. The level of protection offered by vaccination and surviving a previous infection changed during the study period. Vaccination remains the safest strategy for protecting against COVID-19.

There are important caveats to the data presented in this study:
  • The analysis was conducted before the emergence of the Omicron variant and the findings cannot be applied to the current Omicron wave.
  • The study ended prior to the widespread implementation of booster doses and does not reflect the immunologic benefit of additional vaccine doses.
  • The analysis did not include information on the severity of initial infection and does not reflect the risk of morbidity and mortality from COVID-19 infection.
Additionally, a recent study shows with increasing time since prior infection, vaccination provides greater protection against COVID-19 compared to prior infection alone, emphasizing the importance of being up to date on COVID-19 vaccination. Later this week, CDC will publish additional data on COVID-19 vaccines and boosters while Omicron has been circulating.

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We've known for quite some time that coronavirus infections tend to leave behind immunity of limited duration (see Nature Medicine: Seasonal Coronavirus Protective Immunity Is Short-Lastingmaking herd immunity - from natural infection or a vaccine - more difficult to achieve. 

As someone who had COVID before he was eligible for the vaccine, and who has subsequently been vaccinated and boosted, I take this report as encouraging news.  I presumably have the best of both worlds, although their value in preventing Omicron infection is certainly less than against Delta. 

But there is data showing that being vaccinated substantially lowers the risk of severe disease, even when it doesn't prevent infection. So I'll gladly take whatever I can get. 

SARS-CoV-2 is a constantly evolving virus.  And anything that we might say about the impact of vaccination (and boosters), prior infection - or both -  against this virus will undoubtedly change over time.