Now that an independent advisory panel has voted in favor of approving Pfizer’s two-dose Covid-19 vaccine, it likely won’t be long before the US Food and Drug Administration (FDA) clears it for emergency use. Within 24 hours of their decision, millions of doses will be en route to health workers and long-term care residents across the country, beginning a mass vaccination campaign that many hope will end in herd immunity.

A vaccine will make a critical addition to our defenses against Covid-19—this much is certain. But our ability to achieve vaccination-induced herd immunity is less so. While that shouldn’t stop us from trying, it also shouldn’t stop us from practicing and promoting safety guidelines that can actually contain the disease. If Covid-19 is a raging forest fire, and a vaccine the firefighter dispatched to quell it, tried-and-true prevention measures—social distancing, mask wearing, and rapid testing among them—are how we protect ourselves until the trucks come rolling in.

The World Health Organization estimates that 65 to 70 percent of a given population must be vaccinated to halt the spread of disease. Once that threshold is crossed, the Covid-19 virus will have too few human hosts to choose from, driving down transmission rates dramatically. The process of getting there is simple enough in theory, but laborious and time-consuming in practice, involving many factors that can be enumerated endlessly but boiled down to four.

The first factor involves properties intrinsic to both the vaccine and the virus. While the Pfizer vaccine has proven safe and effective at reducing disease in more than 95 percent of clinical trial participants, whether it prevents infection and transmission remains unknown. Likely it will, but until this is evident, we must continue to take great care in protecting ourselves and others from contracting disease, even post-vaccination.

The second factor is the duration of protection—in other words, how long the protection conferred by a vaccine will last. Several studies, though not all, suggest that natural immunity to the Covid-19 virus is quick to fade. Vaccine-induced immunity might do nature one better, but that isn’t always the case. The immune response precipitated by the influenza vaccine, for instance, dwindles in four to six months. Only time will tell whether our protection against Covid-19 is as short-lived.

The third factor—social, rather than biological this time—is how many people take the vaccine. Due to a lack of data on how the Pfizer vaccine affects children and teens, Thursday’s FDA advisory panel deliberated its use in people 16 and over only. It may be that children below the age of 18, who make up about 23 percent of the population, aren’t cleared for vaccination for several months.

Until then, the onus is on adults—some of whom have their own reasons for abstaining. Whether their skepticism is fueled by fear-mongering from the anti-vax movement or, in the case of minority groups and in particular black Americans, a long history of malpractice in the name of medicine, the fact remains that 37 percent of adults in the United States would refuse to get vaccinated if the option were immediately available to them. Though trust, once lost, isn’t necessarily gone forever, it will take a concerted effort led by credible public figures and community leaders to undo damage that deep-seated.

The fourth and final factor is that the pandemic won’t be over in the United States until it’s over everywhere. For the rest of the world, mass vaccination will likely proceed at a much slower pace. If national public health interventions and global vaccine equity initiatives don’t pick up the slack, we can expect an influx of infections to continue streaming in from overseas, whether on the backs of travelers or packaging of frozen foods. Try as they might—and for the most part succeed—at keeping the virus at bay, China has traced hundreds of new infections to contaminated freight and other imported goods. While China has the wherewithal to quash even the smallest, most localized outbreaks with a barrage of testing, in many countries that slow drip is enough to trigger a flood.

Taking these factors into consideration, it becomes clear that vaccination-induced herd immunity isn’t a given. Whether it’s feasible, we probably won’t know for another couple of months. We do know that containing Covid-19 without a vaccine is. Countries like Australia, New Zealand, Thailand, and Taiwan, where strict lockdowns and basic prevention measures were rigorously enforced, have barely a handful of new cases to report daily, if any at all. Vaccines are just one tool among many, and to end the pandemic by this time next year, we need to give it all we got.