It turns out, however, that it’s much more effective in a two-shot regimen. A second dose brings the J&J vaccine to 94% effectiveness against infection and 100% against hospitalization, almost identical to the two mRNA vaccines also approved for use in the US. There is, however, one key difference between them:

A second dose of Johnson & Johnson’s COVID-19 vaccine given two months after the first leads to stronger protection, according to the company.

The new data, announced in a press release, adds to a growing body of evidence suggesting that booster shots could enhance vaccine protection against breakthrough infections — though experts agree all three vaccines are still doing their job to protect against more serious illness.

Compared to the Pfizer and Moderna vaccines, Johnson & Johnson’s single-shot vaccine always had slightly lower efficacy. Peak efficacy from the two-shot Pfizer and Moderna vaccines was 95% and 94%, respectively, against symptomatic illness.

But two Johnson & Johnson shots, given two months apart, resulted in a similarly high effectiveness level: 94% protection against any symptomatic infection in the U.S., and 100% against severe disease.

If this is the case, one has to wonder why J&J didn’t roll out second shots as part of its initial study. They got their EUA at around the same time as Pfizer and Moderna, both of which tested two-shot regimens in that period. The FDA and CDC had high hopes for J&J as a one-and-done vaccine, however, and needed that option for specific targeting of hard-to-reach patients and the vaccine-hesitant. Perhaps that need overrode the obvious scientific approach in the initial EUA-test phase, but it shouldn’t have done so.

Of course, that’s not the only reason why J&J didn’t get around to testing the effect of a booster shot until now. The FDA and CDC all but derailed their vaccine in the US by mishandling anecdotal reports of side effects, completely undermining consumer confidence in it despite its high effectiveness. To some extent, this new data doesn’t help rehabilitate the J&J vaccine even though it should. Most people will still choose the mRNA vaccines, thanks to the aftertaste of the FDA/CDC bungling. This report is really for the 14 million Americans who did take the J&J shot and want to know whether they should get a booster at some point, and for international consumers as well.

That brings us to the key difference between the J&J vaccine and its two other competitors. In the real world data collected by J&J (as well as independent researchers), there is no decline in the one-shot vaccine’s effectiveness over time. It also handles the Delta variant as effectively as the other vaccines, although the CNBC report notes that other variants give it a bit more trouble. The adenovirus platform appears to produce a longer-lasting immune reaction, although that could eventually wear out as well.

Besides, most vaccines require boosters at some point. As a J&J recipient, I have always expected to hear that a booster would be necessary, and have been pleasantly surprised that it’s not been necessary. And it’s apparently still not necessary, but the news that a second shot elevates effectiveness against infection to that level should prompt the FDA and CDC to clear us for a second round of shots ASAP.

After all, the US isn’t having much luck tamping down spikes in cases, even with the relatively high level of vaccinations we have in the US. Why does the US fare so poorly on that score even against countries with similar vaccination levels? David Leonhardt reports that other countries have far more robust rapid-testing capabilities, which the US lacks:

Rapid testing — more formally known as antigen testing — allows people to learn within minutes whether they are carrying enough of the Covid-19 virus to be contagious. With this knowledge, infectious people can stay home and quarantine before they infect others. Everybody else can carry on with life.

In Britain, France and Germany, rapid testing is widely available and inexpensive, thanks to government subsidies. People can visit testing sites, like tents outside pharmacies in France or abandoned nightclubs in Germany, and get tested at no charge. Many people also keep tests in their homes and self-administer them. “It’s been a way to put people’s minds at ease,” Melissa Eddy, a Times correspondent in Berlin, told my colleague Claire Moses.

In the U.S., by contrast, people usually take a different kind of test — known as a P.C.R. test — which must be processed by a laboratory and sometimes does not return results for more than 24 hours. During that time, a person with Covid can spread it to others.

The shortage of testing in the U.S. may be contributing to the virus’s spread. Recent outbreaks have been worse here than in Europe, even though Europe’s vaccination rate is only modestly higher.

Leonhardt quotes Brown professor Stefanie Friedhoff, who experienced the benefits of Germany’s flood-the-zone approach to rapid testing:

It is incomprehensible how the US has failed on testing. Tests approved in other nations are still waiting for FDA approval, some have been for 8+ months. Congress approved billions for testing but no mechanism was created to ensure implementation of the mitigation tool. /11

— StefanieFriedhoff (@stef_friedhoff) September 8, 2021

The testing failure in the US started at the very beginning, when the CDC botched it and then the Trump administration shifted its focus to the vaccines in Operation Warp Speed. While that was a rational but limited choice (we could have done both), there has been no excuse for that failure over the past year and in two administrations. We need cheap, rapid testing until we get to some level of herd immunity — and that’s a lot less costly than locking large percentages of our population out of public life or keeping kids out of school forever.