Vaccines show declining effectiveness against infection overall but strong protection against hospitalization amid delta variant

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Three studies published Wednesday by the Centers for Disease Control and Prevention show that protection against the coronavirus from vaccines declined in the midsummer months when the more contagious delta variant rose to dominance in the United States.

At the same time, protection against hospitalization was strong for weeks after vaccination, indicating the vaccines will generate immune fighters that stave off the worst effects of the virus and its current variations.

Data from these studies persuaded the Biden administration to develop a plan for additional doses to bolster the immune systems of people vaccinated months earlier. The trio of reports, published Wednesday in the Morbidity and Mortality Weekly Report, the CDC’s scientific digest, also reinforce the idea that vaccines alone will be unable to lift the nation out of the pandemic.

Masks and other precautions should be part of “a layered approach centered on vaccination,” wrote researchers from the New York State Department of Health and the University at Albany School of Public Health in their study of vaccine effectiveness across New York state.

All three reports measure vaccine effectiveness, which compares the rates of infection or hospitalization among vaccinated people with the rates among people who had not been vaccinated.

Until now, evaluations of vaccine effectiveness amid delta largely relied on observations from outside the United States. A recent New England Journal of Medicine study concluded the Pfizer vaccine was 88% effective against infections that caused symptoms in England.

Others, such as a study in Israel, found larger declines in protection against infection. One U.S. report that has not yet gone through peer review, collecting data from Mayo Clinic Health System facilities in five states, found a drop in the Pfizer-BioNTech vaccine’s effectiveness against delta infections to 42%. The other mRNA vaccine, made by Moderna, was 76% effective.

The new New York study is the first to assess vaccine protection against coronavirus infection across the entirety of a U.S. state amid delta. The study authors found a modest drop in effectiveness: It descended from 92% in May to 80% in late July. Twenty percent of new infections and 15% of hospitalizations from covid-19, the disease caused by the coronavirus, were among vaccinated people.

The second of the three studies published Wednesday by the CDC found effectiveness against infection declined for nursing home residents after delta emerged. It dropped from 75% in March through May to 53% in June and July. Vaccination for visitors and staff is crucial, the study authors wrote, and “additional doses of COVID-19 vaccine might be considered for nursing home and long-term care facility residents.”

The third report, an analysis of patients at 21 hospitals in 18 states, found sustained protection against hospitalization. Effectiveness was steady at 86%, even in the midsummer months when delta outcompeted other variants of concern. For adults who do not have compromised immune systems, that effectiveness stood at 90%.

Many factors influence vaccine effectiveness. The changes the scientists observed in New York cannot be attributed to delta with certitude, they noted.

If vaccinated humans behave in riskier ways, such as not wearing masks in crowded areas, that may influence vaccine effectiveness. So might waning immune protections. Or, because this is a relative measure, if unvaccinated people acquire immunity through infection, vaccine effectiveness will appear to decline.

To conduct their study, the researchers in New York linked multiple health reporting systems across the state. These included immunization registries, the statewide collection of coronavirus laboratory test results and the system that surveys New York’s inpatient facilities daily. Those databases allowed the study authors to connect vaccine status to every new case and hospitalization reported to the state from May 3 to July 25.

“The New York state data gives us a nice look at how we can link data together when you have comprehensive reporting across a number of systems,” said Robert A. Bednarczyk, an epidemiologist at Emory University Rollins School of Public Health, who has worked with the study authors in the past but was not involved with this research.

By the end of the study period, 66% of New Yorkers 18 and older were vaccinated. Vaccine effectiveness against hospitalization remained constant, above 90%.

Of the more than 48,000 new infections from late spring into summer, 9,675 were in vaccinated people, or about 1 in 5 cases. Breakthrough cases such as these do not mean the vaccines are failing, Bednarczyk said: “The vaccine is doing what it’s supposed to do. It’s priming our immune system.”

Immunized people may still get infected, because the vaccines aren’t perfect. But it is possible immune fighters will sweep the virus out the door much more quickly in a vaccinated person, Bednarczyk said, citing a not-yet-peer-reviewed paper from researchers in Singapore. In that report, vaccinated patients more swiftly defeated an infection compared with those who weren’t.

The results in New York may not easily translate to other communities. Maria Sundaram, an infectious-disease epidemiologist at the University of Toronto Dalla Lana School of Public Health, said it is difficult to make comparisons partly because this is an examination of an entire state, not a model that accounts for uncertainties in a population sample.

The change in vaccine effectiveness over time “to about 10% lower, I would take with a grain of salt,” Sundaram said, because there may be uncertainty from mismatches between databases or reporting lags.

Studies such as these show that, as valuable as coronavirus vaccines are, they have limits. “As we’re releasing the brakes on these other non-pharmaceutical interventions” – meaning masks and other precautions – “we may see more cases,” Sundaram said. “Vaccines are very, very helpful but they’re not the end-all, be-all of covid-19 prevention.”