New US studies released Friday show the Covid-19 vaccines remain highly effective against hospitalisations and death even as the highly contagious delta variant swept the country.

One study tracked over 600,000 Covid-19 cases in 13 states from April through mid-July. As delta surged in early summer, those who were unvaccinated were 4.5 times more likely than the fully vaccinated to get infected, over 10 times more likely to be hospitalised and 11 times more likely to die, according to the US Centers for Disease Control and Prevention (CDC).

"Vaccination works," Dr Rochelle Walensky, CDC's director, told a White House briefing Friday.

But as earlier data has shown, protection against Covid infection is slipping: It was 91% in the spring but 78% in June and July, the study found.

So-called "breakthrough" cases in the fully vaccinated accounted for 14% of hospitalisations and 16% of deaths in June and July, about twice the percentage as earlier in the year.

An increase in those percentages is not surprising: No one ever said the vaccines were perfect and health experts have warned that as more Americans get vaccinated, they naturally will account for a greater fraction of the cases.

Read: Global vaccine disparity gets sharper amid talk of boosters

Rochelle said Friday that well over 90% of people in US hospitals with Covid are unvaccinated.

The CDC released two other studies Friday that signalled hints of waning protection for older adults. One examined Covid hospitalisations in nine states over the summer and found protection for those 75 and older was 76% compared to 89% for all other adults. And in five Veterans Affairs medical centres, protection against Covid hospitalisations was 95% among 18- to 64-year-olds compared to 80% among those 65 and older.

It is not clear if the changes seen over time are because immunity is waning in people first vaccinated many months ago, that the vaccine is not quite as strong against delta – or that much of the country abandoned masks and other precautions just as delta started spreading.

But the US health authorities will consider this latest real-world data as they decide if at least some Americans need a booster, and how soon after their last dose. Next week, advisers to the Food and Drug Administration will publicly debate Pfizer's application to offer a third shot.