Las vacunas de Pfizer y AstraZeneca resisten a la variante delta tras dos dosis

A pair of studies published in Nature and Nature Medicine arrived with reassuring news in mid-July 2021: the vaccines that had begun rolling out across the world—Pfizer, Moderna, and AstraZeneca—were holding up against the delta variant, the strain that was consolidating its grip on Europe. The finding mattered because delta had shown an unsettling ability in laboratory conditions to dodge some antibodies, raising fears that the vaccines might lose their edge just as the pandemic seemed to be turning.

Researchers took serum from 103 people who had been infected with SARS-CoV-2 up to a year earlier, along with samples from 59 volunteers who had received one or two doses of either Pfizer or AstraZeneca. They also tested laboratory-made monoclonal antibodies used in treatments like Regeneron's cocktail. Then they introduced the delta variant, designated B.1.617.2, and waited to see what would happen. Oliver Schwartz, a researcher at the Pasteur Institute and one of the study's authors, explained that delta's increased transmissibility came paired with a partial resistance to neutralization by antibodies. The antibodies from people who had been infected in the past proved four times less potent against delta. For those vaccinated with a single dose of either Pfizer or AstraZeneca, the results were stark: the serum barely inhibited delta and beta variants, with only 10 percent of samples capable of neutralizing the virus at all.

But the critical finding was what happened after the second dose. Two shots of either vaccine produced a neutralizing response in 95 percent of individuals tested—a threshold that suggested real protection. The antibody levels were weaker than they had been against the original alpha strain, dropping to between three and five times less potent, yet the vaccines still held. Schwartz cautioned that a single dose did not activate sufficient quantities of neutralizing antibodies, making the completion of the vaccination series essential. The message was clear: one shot left people vulnerable; two shots restored the shield.

Meanwhile, concern was building around a new mutation that had emerged in India. The government there had detected what became known as delta plus, also labeled AY.1 or B.1.617.2.1, carrying an extra mutation in the spike protein of the virus. This variant had been found in 48 infected people among more than 45,000 samples sequenced in India. Sunit K. Singh, a researcher at Banaras Hindu University, warned that delta plus might possess a similar capacity to evade immunity and could reduce the effectiveness of monoclonal antibody therapies used to treat severe COVID-19. The mutation's location was troubling—it sat in the spike protein itself, the key structure the virus uses to penetrate human cells. Previous mutations had clustered in the receptor-binding domain, the part that allows the virus to attach to our cells; this new one was positioned differently, in territory that had not been heavily mutated before.

The delta variant's own mutations had already demonstrated an ability to partially escape the immune system, reducing vaccine effectiveness somewhat. A single dose offered diminished protection. But the Nature study showed that a second dose generated enough antibodies to prevent both symptomatic infection and severe disease. Singh noted an important distinction: most COVID-19 vaccines did not provide absolute sterilizing immunity—the kind that prevents infection entirely—but rather worked to reduce how sick people became. Delta plus might show a similar pattern of reduced vaccine effectiveness, though solid data on that question did not yet exist. Studies were underway in India to test the vaccines against this new variant.

Delta plus had not yet spread substantially, and the World Health Organization had not classified it as a variant of concern. Schwartz added a crucial caveat: the Nature study had focused narrowly on how the variant affected antibodies. Real-world effectiveness data had shown that the immune system operated far beyond antibodies alone. The cellular immune response—the other major arm of immunity—played an important role too. Meanwhile, separate findings published in Nature Medicine showed that Moderna's vaccine was 100 percent effective against the alpha variant and 96 percent effective against beta, measured 14 days after the second dose. Qatar had provided a natural experiment: vaccination began there on December 28, 2020, but mass vaccination did not start until late February 2021. As the campaign expanded, the country experienced two consecutive waves of infection driven by the introduction of alpha and beta variants, allowing researchers to measure real-world vaccine effectiveness against these strains.

The vaccines had reversed a pandemic in which despair had spread as widely as the virus itself. The arrival of variants had rekindled old fears that these tools, which were saving so many lives, might stop working. For now, data from countries like the United Kingdom and Israel offered reassurance: the vaccines were holding.