Three major federal studies released Friday provide the clearest U.S. evidence yet that COVID-19 vaccines continue to protect strongly against hospitalization and death during the Delta surge, even as protection against infection appears to have weakened. Across 13 jurisdictions, unvaccinated adults were about 10 times more likely to be hospitalized and roughly 11 times more likely to die than fully vaccinated adults during the period when Delta became dominant.

The largest analysis, an early-release CDC report, examined more than 600,000 cases, nearly 38,000 hospitalizations and more than 6,700 deaths between April 4 and July 17. After Delta became predominant, the age-standardized rate of reported infection among unvaccinated adults was about 4.6 times the rate among fully vaccinated adults; hospitalization was about 10.4 times higher and death about 11.3 times higher.

Those results arrived one day after President Biden announced a broad new vaccination strategy, but the studies stand on their own as a large body of real-world evidence. CDC Director Rochelle Walensky said during a September 10 briefing that vaccination remained the country’s most important tool for preventing severe complications.

Protection against severe disease is holding up

The first study divided its data into two periods: one before Delta accounted for at least half of sequenced virus and one after. The relative protection against reported infection declined as Delta spread, but the gap in hospitalization and death between vaccinated and unvaccinated adults remained large.

That distinction is important. Vaccines were never expected to prevent every infection, and breakthrough cases have become more visible as Delta has spread and the vaccinated share of the population has increased. The central question for hospitals and public-health officials is whether vaccination still prevents the outcomes that place the greatest burden on patients and the healthcare system.

By that measure, the new data are reassuring. The report found only modest change in the relative rates of hospitalization and death during Delta’s rise. It also found that vaccinated adults represented a minority of severe outcomes even as vaccination became common across the country.

The CDC’s public summary of the same findings was released as a one-page graphic emphasizing that fully vaccinated people had substantially lower risks of infection, hospitalization and death after Delta became the most common variant. That communication reflects a shift in the agency’s message toward the outcomes most clearly preserved by vaccination.

Different vaccines show different estimates, but all provide substantial protection

A second CDC study looked at more than 32,000 emergency-department, urgent-care and hospital encounters across nine states during June through August. The VISION Network analysis estimated that vaccination overall was 86 percent effective against hospitalization during Delta predominance.

The estimates differed by product. Moderna’s vaccine was calculated at 95 percent effectiveness against hospitalization, Pfizer-BioNTech at 80 percent and Johnson & Johnson at 60 percent. Against emergency-department and urgent-care encounters, estimated effectiveness was 92 percent for Moderna, 77 percent for Pfizer and 65 percent for Johnson & Johnson.

Researchers cautioned against treating those numbers as a simple ranking. The groups receiving each vaccine differed in age, geography and timing of vaccination, and observational studies cannot fully eliminate every source of confounding. Still, the pattern gives health officials a reason to continue monitoring whether product, dosing interval or time since vaccination affects durability of protection.

Regenstrief Institute, one of the organizations contributing data to the VISION Network, said in a contemporaneous research summary that the findings show vaccines remain highly effective at reducing hospitalization and emergency care even with Delta circulating widely.

Veterans Affairs data reinforce the hospitalization finding

A third study focused on adults hospitalized at five Veterans Affairs medical centers in Atlanta, Houston, Los Angeles, New York and Palo Alto. The VA analysis estimated that two-dose mRNA vaccination was 87 percent effective against COVID-19 hospitalization from February through early August.

Effectiveness was estimated at 95 percent for adults ages 18 to 64 and 80 percent for those 65 and older. The lower estimate among older adults is one of several signals that protection may be less durable in age groups vaccinated earlier and at higher baseline risk.

The combined picture is therefore more nuanced than either “vaccines stopped working” or “nothing has changed.” Protection against becoming infected has declined from the exceptionally high levels seen in the first months of the vaccination campaign, particularly during Delta. Protection against severe disease remains much stronger.

A same-day Washington Post report noted that the three studies together address more than one question: whether vaccines still prevent severe outcomes, whether effectiveness differs among products and whether older adults show signs of waning protection.

The next policy question is boosters, but the data are not a final answer

The findings arrive as federal regulators prepare to consider booster doses, beginning with Pfizer-BioNTech. The evidence of sustained protection against hospitalization and death complicates the debate. If vaccines remain highly effective at preventing severe illness for most adults, regulators must decide whether declining protection against infection alone justifies broad booster use.

The data also support a more immediate conclusion: the largest preventable risk remains among people who have not completed an initial vaccine series. An Axios summary of the CDC report emphasized that only a small share of the hospitalizations and deaths in the 13-jurisdiction analysis occurred among fully vaccinated people.

Public-health officials must still monitor breakthrough infections, duration of protection, variant evolution and the possibility that effectiveness against severe disease could decline with time. The new studies cover periods ending in July or early August, so they do not yet capture the full late-summer Delta wave or the longest intervals since vaccination.

For clinicians and patients deciding what the evidence means today, however, the signal is consistent across three different datasets: vaccination sharply reduces the probability that a coronavirus infection will become a hospitalization or a death. Delta has changed the pandemic’s transmission dynamics, but the vaccines remain a powerful barrier against its worst clinical outcomes.