Three studies published by the Centers for Disease Control and Prevention (CDC) on Wednesday show that protection against infection from the coronavirus in vaccinated individuals has declined over time.
The studies were among the evidence that officials used to recommend booster vaccine doses.
The agency made clear that the currently authorized vaccines still have very high effectiveness against COVID-19 hospitalization, meaning that even if someone gets infected after being vaccinated, they will generally not be seriously ill.
"The data we will publish today and next week demonstrate the vaccine effectiveness against SARS CoV-2 infection is waning," CDC Director Rochelle WalenskyRochelle WalenskyOvernight Health Care — Presented by Indivior — CDC panel approves boosters for some, but not based on jobs CDC panel authorizes COVID-19 vaccine boosters for high-risk people, those over 65 FDA authorizes Pfizer COVID-19 booster shot for older and high-risk Americans MORE said during a press briefing. "And even though our vaccines are currently working well to prevent hospitalizations, we are seeing concerning evidence of waning vaccine effectiveness over time, and against the delta variant."
All three reports measured the level of vaccine effectiveness, which compares the rates of infection or hospitalization among vaccinated people with the rates among unvaccinated people.
But the CDC studies were not able to distinguish whether the drop in effectiveness against infections was a result of the delta variant, because of people changing their behaviors and the relaxing of masking and distancing requirements, or a real drop in immunity.
Data from the three reports, published Wednesday in the CDC's Morbidity and Mortality Weekly Report, helped convince the Biden administration of the need to recommend booster shots to people eight months after receiving their second dose.
Under the plan announced Wednesday, boosters will start being administered Sept. 20, pending authorization of a third dose from the Food and Drug Administration and a CDC advisory committee meeting to make recommendations based on the evidence.
"We are concerned that this pattern of decline we are seeing will continue in the months ahead, which could lead to reduced protection against severe disease hospitalization and death," Surgeon General Vivek MurthyVivek MurthyCDC panel authorizes COVID-19 vaccine boosters for high-risk people, those over 65 FDA authorizes Pfizer COVID-19 booster shot for older and high-risk Americans GOP senators say Biden COVID-19 strategy has 'exacerbated vaccine hesitancy' MORE said.
Health experts said the CDC data should make the case that it's more important to get initial doses to the unvaccinated, and boosters to immunocompromised people and nursing home residents, rather than to the entire population.
"I mostly care about hospitalizations, I don't care about infections because this is not what we're using vaccines for. We're not trying to stop infections, and there's no evidence that a third booster will stop infections," said Jennifer Nuzzo, an epidemiologist and senior scholar at the Johns Hopkins Center for Health Security.
One study examined vaccinated and unvaccinated people in New York from May 3 to July 25. According to the study, the overall age-adjusted effectiveness against new COVID-19 cases for all adults declined from 91.7 percent to 79.8 percent.
During the same period, the effectiveness against hospitalization was relatively stable, ranging from 91.9 percent to 95.3 percent.
Nuzzo said those numbers don't show a compelling reason to give a third dose, especially because a second study found that the vaccines still showed 90 percent effectiveness against hospitalizations.
"What could be perceived as the vaccine not protecting as much as it did before based on time could just be due to the fact that we're challenging the vaccine more than we did before," Nuzzo said.
"We need to remember that vaccines aren’t force fields — they don’t prevent infections. They train your immune system to respond quickly to infections and hopefully limit the number of cells that get infected. They work to limit infections to prevent severe disease, hopefully to keep people out of the hospital," she added.
The study suggested that vaccines alone won't be enough to reduce new COVID-19 cases and hospitalizations. The findings suggested a layered approach that includes vaccines, as well as other prevention strategies such as masking and physical distancing.
One area where boosters could be helpful is among nursing home residents, who are often elderly and frail and might have a less robust response to vaccines.
A third CDC study showed vaccine effectiveness in nursing homes has been significantly lower in more recent months.
The study analyzed the effectiveness of vaccines among residents of nearly 4,000 nursing homes from March 1 to May 9, before the delta variant’s emergence, and nearly 15,000 nursing homes from June 21 to Aug. 1, when delta was the primary variant driving new infections in the country.
The effectiveness dropped from about 75 percent to 53 percent during the period of delta variant circulation.
"It makes sense to give an extra dose of vaccine to vaccinated nursing home residents, but what will have an even bigger impact on protecting those nursing home residents is to vaccinate their caregivers," tweeted Celine Gounder, an infectious diseases doctor at Bellevue Hospital Center and former adviser to the Biden campaign on COVID-19.
However, CDC researchers said they couldn't determine if the drop in effectiveness was just from the delta variant or a combination of delta and waning immunity. The study also did not evaluate the vaccines’ protection against severe illness.