Three US studies suggest COVID-19 vaccines offer strong protection against hospitalisation and death, even in the face of the highly transmissible Delta variant, but vaccine protection appears to be waning among older populations, especially among those 75 and older.

US data on hospitalisation from nine states during the period when the Delta variant was dominant also suggests that the Moderna Inc vaccine was more effective at preventing hospitalisations among individuals of all ages than vaccines from BioNTech/Pfizer Inc or Johnson & Johnson .

In that study of more than 32,000 visits to urgent care centers, emergency rooms and hospitals, Moderna’s vaccine was 95% effective at preventing hospitalisation compared with 80% for Pfizer and 60% for J&J.

Overall, the findings, released on Friday in the US Centers for Disease Control and Prevention’s (CDC) weekly report on death and disease show that vaccines continue to offer strong protection from COVID-19.

One of the studies involved more than 600,000 COVID-19 cases, hospitalisations and deaths in 13 states and large cities from April through mid-July.

It found that during the past two months, a period that includes the impact of the Delta variant, unvaccinated individuals were about 4.5 times more likely to get COVID-19, 10 times more likely to be hospitalised and 11 times more likely to die from the disease than those who were fully vaccinated.

In a White House COVID-19 briefing on Friday, CDC Director Dr. Rochelle Walensky said the data show that “vaccination works and will protect us from the severe complications of COVID-19.”

While protection against hospitalised disease and death remained strong against Delta, the study also confirms an increase in milder COVID-19 infections among fully vaccinated people, which the authors said reflected “potential waning of vaccine-induced population immunity.”

Another study looked specifically at the performance of mRNA vaccines – such as the shots from Pfizer and Moderna – in patients at five Veterans Affairs medical centers, a racially diverse group made up largely of older male patients with higher rates of underlying disease.

Of the more than 1,000 COVID-19 hospitalisations in that study, researchers found that combined both vaccines were 86.8% effective against hospitalisation – even against the Delta variant. But vaccine effectiveness fell to 79.8% among veterans 65 and older.

The third study, which looked at medical encounters in nine states, overall vaccine effectiveness remained high at 86% against hospitalisation and 82% against visits to the emergency room or an urgent care center. However, vaccine effectiveness against hospitalisation was “significantly lower” among adults aged 75 and older, falling to 76% – the first time a drop had been observed in this data set.