In Homer’s “Iliad,” the story of the war between the ancient Greeks and Trojans, the Greeks used a secret weapon, the Trojan horse, to vanquish their enemy at the close of a brutal 10-year conflict. In the battle against COVID-19, our own Trojan horse is vaccination.

Cases and deaths have fallen dramatically since their peaks in the winter. But just as the Trojan horse did not end the threats to the Greeks, which included a pandemic during a subsequent war with Sparta, we are not out of danger either.

The United Kingdom and Israel, both with higher percentages of vaccinated populations than the U.S., have seen case surges in the unvaccinated, as well as rare breakthrough infections in those vaccinated. Israel, where cases had been as low as 25 a day, is averaging 10 times as many new cases, and the country has reinstituted masking. Likewise, the U.K. is now averaging 10 times as many cases as it did during its nadir.

As long as there is a large enough pool of unvaccinated people, viral variants will emerge. The most notable is the delta variant, first discovered in India, which spread across the globe and is responsible for the current surges in Israel and the U.K. Vaccination remains in a deadly race with the emergence of these extremely contagious variants. The good news is that the variants are susceptible to the vaccine. In Israel and the U.K., vaccination has limited transmission and simultaneously suppressed most, but not all, of the deaths and hospitalizations in both countries.

In the U.S., vaccination has been amazingly effective at protecting older, high-risk individuals, but our problem lies with the unvaccinated, especially young adults who may believe the pandemic is petering out. Adults ages 18 to 29 have the lowest vaccination rate of any age group in the U.S. Fortunately, younger adults who are infected have fewer hospitalizations and lower mortality rates, although the problem of “long-haul” COVID-19 residual symptoms remains a risk. There is also the danger of spreading the virus to other unvaccinated people or to immunocompromised patients.

Cases are already rising in parts of the U.S. where vaccination rates are low and the delta prevalence is high.

What can we do? There is no good option but to vaccinate as many as quickly as possible, ensuring vaccination of all those eligible in the U.S. and ultimately of the entire planet’s population.

Now we are faced with another nettlesome public health dilemma: whether to mandate vaccination of children for school attendance, as we do for other childhood vaccines. For the safest school environment for children, vaccination must be mandatory for school staffs, who are adults in whom the vaccine has been extensively tested. Discussions should begin now on what it will take to open in the fall. And mandated vaccination for the young is on the docket.

The U.S. Centers for Disease Control and Prevention has little choice but to issue some directive before the school year starts. As it stands now, encouraging parents to do the sensible might well have greater success at getting kids vaccinated than ordering parents to do the sensible thing.

Dr. Cory Franklin is a retired intensive care physician. Dr. Robert Weinstein is an infectious disease specialist at Rush University Medical Center. The two worked together in Chicago during past flu outbreaks and the AIDS epidemic.