Third COVID vaccine dose boosts effectiveness against hospitalization
A third COVID-19 vaccine dose increased vaccine effectiveness (VE) against hospitalization from 82% to 97% among adults with healthy immune systems and from 69% to 88% among those with impaired immunity, according to a study today in Morbidity and Mortality Weekly Report (MMWR).
Led by researchers from the Centers for Disease Control and Prevention (CDC) COVID-19 Emergency Response Team, the study involved 1,385 COVID-19 patients and 1,567 uninfected controls admitted to 21 US hospitals from Aug 19 to Dec 15, 2021. The study period encompassed the emergence and eventual dominance of the highly transmissible Delta (B1617.2) variant.
The researchers estimated the VE of a third dose of the Pfizer/BioNTech and Moderna mRNA vaccines against COVID-related hospitalization in 312 adults who received a third shot at least 7 days before symptom onset, relative to 1,251 adults who hadn't received a third dose. They estimated VE using logistic regression, comparing the chances of being vaccinated versus unvaccinated among COVID-19 patients and controls. Median patient age was 62 years, 49% were women, and 58% were White.
Of 1,875 participants with uncompromised immune systems, 57% were unvaccinated against COVID-19, 36% had received two doses of vaccine, and 7% had been given a third dose. VE against COVID-19–related hospitalization in this group was 97% among participants given a third dose (95% confidence interval [CI], 95% to 99%), compared with 82% (95% CI, 77% to 86%) among two-dose recipients.
Of 1,077 adults with immune-compromising conditions, 30% were unvaccinated, 53% had received two doses, and 17% had received a third dose. VE was 88% (95% CI, 81% to 93%) among third-dose recipients, versus 69% (95% CI, 57% to 78%) among those who had received two doses.
The study authors noted that the study was conducted before the emergence and eventual dominance of the Omicron (B.1.1.529) variant. COVID-19 VE, they said, may now be lower than for other variants because Omicron may be able to evade immune protection, but early data show some promise.
"Early evidence suggests that a third mRNA vaccine dose elicits markedly stronger neutralizing antibody responses to the Omicron variant compared with responses to 2 vaccine doses, and increases VE against severe disease following infection with the Omicron variant," they wrote.
In October, the CDC began recommending a three-dose primary regimen plus a fourth dose as a booster for those with impaired immune systems.
Jan 28 MMWR study
Studies show Omicron variant less severe but causes more reinfections
Two studies published yesterday in Eurosurveillance show the Omicron variant leads to fewer hospitalizations than the Delta variant, but an increased risk of infection in vaccinated and previously infected people.
The hospitalization study was based on medical records in Norway. Overall, 39,524 Omicron (43%) and 51,481 Delta cases (57%) were included in the study, with positive tests collected from Dec 6, 2021 to Jan 9, 2022.
The authors found Omicron carried an overall 73% reduced risk of hospitalization (aHR = 0.27; 95% confidence interval [CI], 0.20 to 0.36) compared with Delta.
The authors said this finding confirms data from South Africa, the United Kingdom, Canada, and the United States, which have thus shown Omicron is associated within 36% to 66% reduced risk of hospitalization compared with Delta.
In a second study from the Netherlands, the authors used confirmed COVID-19 cases collected from Nov 22, 2021, to Jan 19, 2022, to show Omicron carried an increased risk of infection compared with Delta in people with COVID-19 immunity—either from previous infections or vaccination.
Based on 174,349 cases, 66.1% of patients who had Omicron had received a primary vaccination (two dose) series, compared with 61.5% of Delta patients. Also, 6.5% infected with Omicron had a previous infection, compared with 1.4% of Delta.
"We showed that individuals with a previous infection had a higher risk of infection with Omicron than with Delta compared with naïve individuals, suggesting that previous infection with another SARS-CoV-2 variant provides lower levels of protection against Omicron than against Delta infection," the authors wrote.
Jan 27 Eurosurveill Norway study
Jan 27 Eurosurveill Dutch study