Last week the CDC authorized giving a booster shot to Americans who had conditions that impaired their immune systems. Many people in the midst of cancer treatment, or taking certain prescription drugs just didn’t develop the necessary immune response for the standard dosage and needed an extra jolt to get to a reasonable level of protection, particularly against the highly contagious delta variant.

But just one week later, it seems that the CDC is expanding the pool of who can get a booster shot by a factor of infinity. As in booster shots may be available to everyone as early as next month.

As The Washington Post reports, the Biden administration is expected to announce that those who have already received COVID-19 vaccinations will need a booster shot eight months after that first injection. Part of the reason for this is that, since the first trials of the vaccines were complete, those people—American and elsewhere—who participated in those trials have been tracked to see how signs of a strong immune reaction were holding up. And while it seemed a few months ago that the vaccine response was going string, there has seen been evidence of a decline in vaccine benefits for those earliest recipients. For most people, the immune response at eight months is likely to be still very good, but the idea is to make sure it stays that way, rather than leaving a window of vulnerability.

The other reason for the booster shot is directly related to that growing spike of delta variant cases in the U.S., and to data that recently emerged from Israel. In a report which appeared in Science on Monday, the data suggest that a booster may be required to break the chain of transmission for the highly contagious variant.

Israel’s vaccination rate is among the highest in the world, with 78% of those 12 and older fully vaccinated. That compares to 59% in the United States. Most Israelis were vaccinated with the Pfizer vaccine which, until recent days, has been considered perhaps the most effective vaccine against COVID-19.

Despite all this, Israelis seeing an incredible increase in new cases of COVID-19. As noted by WorldOMeters, the nation is near its all-time high in terms of new cases, and nearly half those cases are among vaccinated people. In fact, of 514 hospitalized, 59% were fully vaccinated. 

This doesn’t mean that the vaccine has become ineffective against delta, at least in terms of providing protection to the vaccinated. In a nation where so large a percentage of people are vaccinated, and case is much more likely to be a “breakthrough” case. Even though the unvaccinated make up 21% of the population, they’re still 41% of those hospitalized. The Pfizer vaccine continues to provide good protection against severe illness from COVID-19, including the delta variant—but not nearly as good as expected a few weeks ago. Additionally, that vaccination is clearly not enough to prevent  a vaccinated person from becoming infected, to halt the virus from replicating in a vaccinated person, or to stop a vaccinated person from passing on the virus to others.

And that’s one of the reasons that vaccine booster shots are being pushed forward, in spite of continued good results when it comes to protecting the health of the vaccinated—without an even stronger vaccine response, it may be impossible to break the chain of transmission. Keeping the delta variant down is going to take more than keeping people from getting seriously ill; it means keeping them from being seriously contagious.

In short, people can stop looking for the variant that might outrun the vaccines, at least in terms of prolonging the pandemic. That variant is already here. (As a side note, don’t expect to see any news stories in the short term about how some other variant is taking over, as delta continues to spread so freely and rapidly.) So booster shots are needed to both improve safety for the vaccinated, and control the spread of delta.

However, there is more in the preprint study behind this report than just those fearful topline numbers, and part of what the study shows is both good news for those who are might worry that their vaccine benefits are running low, as well as explain why breakthrough hospitalizations remain much rarer in the United States. Israel began their vaccine push early, and achieved a high rate of penetration in the first few months of vaccine available. 

Those vaccinated in April—which was the peak month for vaccinations in the United States—are less than half as likely to experience a breakthrough infection by delta than those who were vaccinated four months earlier in January.  That relationship may not be as perfect as it seems at first, because the first Israelis to be vaccinated were those most at risk, including those over 80, whose immune systems may not have provided a strong response. But it certainly suggests that while the vaccines are providing a continuing benefit, that benefit needs exactly what the CDC is about to order — a booster.

Just seven million Americans were vaccinated before January 30, most of them either elderly in extended care facilities or healthcare workers. It makes sense that those workers should be back at the front of the line for booster shots. But it was after that point that the real surge in vaccinations began. Despite vaccine hostility among Republicans, by the end of May, over 142 million Americans were fully vaccinated. To get all those people a booster shot over the same period is going to take, one again, a massive effort to set up vaccination locations and staff necessary to process that third jab.

Meanwhile, the World Health Organization has asked that wealthy nations—who have control over the vast majority of the vaccine supply, especially when it comes to the most widely regarded vaccines — should hold off on booster shots. That request is likely to be disregarded in light of the information coming from Israel and other preliminary data under consideration by the CDC.

At this point, it seems likely that people will be getting another dose of the same vaccine they received previously, though as Clinical Trials Arena noted back in April. there is some evidence that mixing more than one type of vaccine could provide extended protection. Earlier this month, a preprint study from the Mayo Clinic, suggested that the Moderna vaccine may be more effective than the Pfizer vaccine when it comes to halting infections by the delta variant.  That could increase demand for this vaccine when it comes to the booster round.

What all of this evidence shows is that, considering the high transmissibility of the delta virus, it’s going to take vaccines, and masks, and good social distancing guidelines to drive down the rate of transmission. Just a few weeks ago, a drop in cases in the UK suggested that a combination of a well-vaccinated population and a number of new cases in the population might have brought that nation to “herd immunity.” However, since then cases have leveled out at nearly 30,000 cases a day in a nation whose population is just over a quarter of that in the United States.

Delta is not going down easy. Anywhere.