This Vaccine Drops in Efficacy After 4 Months, New CDC Study Says

There was no significant decline reported for the other mRNA vaccine.

Over the past few months, we've heard reports of vaccinated people becoming infected with COVID, despite having received their requisite doses. The Centers for Disease Control and Prevention (CDC) has long warned us that breakthrough cases are possible, as no vaccine is 100 percent effective. But through new research, the agency has found that there could be something else leading to more vaccinated people getting COVID: a decline in vaccine effectiveness over time.

RELATED: This Vaccine Protects You Most Against Severe COVID, New CDC Study Says.

The CDC compared the effectiveness of Moderna's, Pfizer's, and Johnson & Johnson's vaccines against COVID hospitalization among adults without immunocompromising conditions in a Sept. 17 study. The researchers analyzed nearly 4,000 patients who were admitted to 21 U.S. hospitals across 18 states from March 11 to Aug. 15 of this year, 36 percent of whom were fully vaccinated with one of the three vaccines.

According to the study, Pfizer's effectiveness against hospitalization declined significantly after 120 days, or nearly four months. The vaccine was initially 91 percent effective, but dropped to 77 percent at the four-month marker.

The researchers did not find a similar drop for Moderna's vaccine, however. According to the study, this vaccine was initially 93 percent effective against COVID hospitalization and only dropped to 92 percent after four months. Johnson & Johnson's vaccine was not available for survey after four months, but just 28 days after full vaccination, its vaccine protectiveness against severe COVID was significantly lower than the other two, at 68 percent.

Moderna also had a higher vaccine effectiveness against hospitalization than Pfizer overall, according to the study. The researchers found that Moderna was 93 percent effective against COVID hospitalizations, while Pfizer was 88 percent effective. On the other hand, Johnson & Johnson was only 71 percent protective against hospitalization.

"[Vaccine effectiveness] against COVID-19 hospitalization was slightly lower for the two-dose Pfizer-BioNTech vaccine than the Moderna vaccine, with this difference driven by a decline in [vaccine effectiveness] after 120 days for the Pfizer-BioNTech but not the Moderna vaccine," the study authors stated.

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Despite Moderna and Pfizer both using mRNA technology for their vaccines, the CDC researchers say there are some potential reasons as to why they produce different levels of protection. According to the study, difference in vaccine effectiveness between the two may be a result of Moderna's vaccine having higher mRNA content (100 micrograms versus Pfizer's 30 micrograms) or a difference in timing between doses (four weeks for Moderna's versus just three weeks for Pfizer's). "Possible differences between groups that received each vaccine that were not accounted for in the analysis" could also explain the study's findings, according to the researchers.

Ultimately, however, all three vaccine are still deemed effective in their protection. "Although these real-world data suggest some variation in levels of protection by vaccine, all FDA-approved or authorized COVID-19 vaccines provide substantial protection against COVID-19 hospitalization," the study authors reiterated. "Current guidelines from FDA and CDC recommend vaccination of eligible persons with one of these three products, without preference for any specific vaccine."

RELATED: Moderna Now Says Vaccine Protection Goes Down After This Long.

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