The Centers for Disease Control and Prevention (CDC) said this week that third shots of coronavirus vaccines significantly reduced the risk that people who are immunocompromised would be hospitalized due to COVID-19.

In a Thursday study in the agency’s Morbidity and Mortality Weekly Report, the agency said that – compared to hospitalized adults who had received two mRNA vaccine doses – the effectiveness of the Pfizer-BioNTech and mRNA vaccines against hospitalization for patients with weak immune systems increased to 88%. 

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For immunocompromised people who had only had two doses, or a “primary series,” the vaccines were 69% effective. 

The data was gathered looking at nearly 3,000 – 1,385 case-patients and 1,567 non-COVID-19 controls – at adults admitted to 21 hospitals across 18 states from Aug. 19 through Dec. 15 of last year.

The agency noted the analysis start date was one week after the Food and Drug Administration (FDA) authorized an additional dose for emergency use for people aged 12 and older with immunocompromising conditions 28 days following their second dose.

Vaccine effectiveness was calculated for both groups by comparing the odds of previous vaccination between COVID-19 case-patients and control patients who did not have the disease. The regression model was adjusted for admission date, region of hospital, age group, sex and self-reported race and ethnicity. Separate models were generated for immunocompetent adults and adults with immunocompromising conditions and analyses were conducted using Stata software.

Those who had COVID-19-like illness received positive test results by a nucleic acid amplification test (NAAT) or antigen test and control patients received negative NAAT tests.

Patients or their proxies were interviewed regarding demographic and clinical characteristics and medical record searches were conducted. Information regarding receipt of prior vaccine doses was obtained through self-report and review of source documentation. 

Three vaccination groups were considered including unvaccinated patients, two-dose mRNA recipients and three-dose mRNA recipients.

Three doses of the Pfizer-BioNTech or Moderna vaccines were roughly 97% effective in preventing hospitalizations in those with stronger immune systems, compared with 82% of those with two doses.

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Notably, the study was conducted when the delta variant was predominant in the U.S., whereas omicron now makes up nearly 100% of new cases nationally.

Limitations to these findings include that vaccine recipients in both dose groups were similar in terms of most demographic and clinical characteristics but may have varied with respect to exposure risk for infection, that vaccine effectiveness was not assessed against mild illness or infection, that vaccine effectiveness with a fourth mRNA vaccine dose in immunocompromised individuals was not assessed and that most three-dose mRNA vaccine recipients were vaccinated within several weeks of enrollment and durability of protection will require future analysis. 

“Early evidence suggests that a third mRNA vaccine dose elicits markedly stronger neutralizing antibody responses to the omicron variant compared with responses to two vaccine doses,  and increases [vaccine effectiveness] against severe disease following infection with the omicron variant,” the CDC wrote. “The effectiveness of three doses of COVID-19 mRNA vaccines against a range of disease severity associated with the omicron variant needs to be carefully evaluated in different populations.”

In January, regulators said adults could receive boosters at least five months after their second dose.

As omicron spread, the White House has called on Americans to get their booster and the CDC said that administration of a third COVID-19 mRNA vaccine dose as part of a primary series among immunocompromised adults, or as a booster dose among immunocompetent adults, provides improved protection against COVID-19 hospitalization.

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Among adults with and without immunocompromising conditions who were eligible to receive a third dose of an mRNA vaccine, third doses were found to increase protection beyond that of a two-dose vaccination series for the prevention of COVID-19 hospitalization. 

According to the American Medical Association, there are about seven million people with weakened immune systems in the country.