Biden had received a second booster. Here’s why it wasn’t enough to prevent infection.

President Biden’s coronavirus infection is a clear example that COVID vaccines, as powerful as they are, are far from the bulletproof shields scientists once hoped for.

Mr. Biden has received multiple doses of the Pfizer-BioNTech vaccine; His most recent shot, the second booster, was on March 30. Studies suggest those doses will provide a powerful shield against serious illness — and in fact, the president has only mild symptoms after testing positive on Thursday, according to the White House.

But even booster doses provide little defense against infection, especially with the most recent versions of the virus. Numerous studies have shown that the little protection they provide wears off faster and faster. In the president’s case, the booster shot he received about four months ago is likely to lose much of his power in stopping the infection.

Earlier in the pandemic, experts believed that vaccines would be enough to prevent not only serious disease, but most infections. And this was true when earlier versions of the virus, including the delta version, spread around the world.

But the Omicron version dashed those hopes. As more populations gained some immunity, whether from infection or vaccines, the virus evolved to dodge those defenses. BA.1, a subtype of Omicron, which circulates in winter, was able to cause infection even in people who had received a booster dose of the vaccine a few weeks earlier.

Each subsequent incarnation of the virus has become still better at eliciting immunity. BA.5, which now accounts for about 80 percent of cases in the United States, is by far the smartest. Detailed data collected in Qatar shows that immunity from previous infections and vaccines is weaker with BA.5 than its predecessors.

BA.5 is also highly contagious. The country is reporting an average of 130,000 cases per day; This number is likely to be very low, as most people test at home or do not test at all.

The number of hospitalizations has also increased over the past few weeks, although BA.5 does not appear to cause more severe disease than other forms of omicron.

Given how much the virus has changed, the administration is debating the value of authorizing additional shots of the original vaccine in the fall and offering a second booster to adults under age 50. An advisory panel to the Food and Drug Administration said last month that vaccine manufacturers should tailor the shots to the latest variants.

But it is not clear whether those shots will arrive in time to stop the fall, and whether the virus will develop beyond their reach once again.