The United States' coronavirus vaccine roll-out has finally hit its stride, with well over two million doses administered daily.
Soon, vaccines will be available to all adults who want them. Children are the next vaccination frontier.
When it comes time to vaccinate them, the same urgency and large-scale coordination efforts driving adult vaccination must continue if we want to sustainably drive down Covid-19 cases and ultimately end the pandemic.
Currently, vaccine demand among adults exceeds the supply. But there's reason to worry that once children are eligible, vaccination rates for them will initially be far lower and rise more slowly than those seen among adults.
Children are much less likely than adults to be hospitalised with Covid-19, and deaths from the disease among kids are rare. Parents may wonder: If Covid-19 is relatively harmless for my children, what's the hurry?
Risk of mutations
One reason to vaccinate children quickly is that even a small number of critical Covid-19 cases among children is worth vaccinating against. The burden of long-term effects from Covid-19 in children - including rare but serious cases of inflammatory syndrome - remains unclear, especially since many have asymptomatic infections that go undiagnosed.
But the most important and least recognised reason to vaccinate all children quickly is the possibility that the virus will continue to spread and mutate into more dangerous variants, including ones that could harm both children and adults.
Variants "of concern" first identified in Britain, South Africa, Brazil and California are being closely followed by epidemiologists. Some of these appear more contagious than earlier versions, and at least one of them - B117, first observed in Britain - appears to cause a slight uptick in the risk of dying from Covid-19. So far, the vaccines still appear to work well against them.
But we might not be so fortunate with future variants. Viruses acquire mutations as they spread. The more infections there are, the more chances the coronavirus has to mutate. This increases the likelihood that a more dangerous strain could emerge.
Variants that cause more severe illness in children are likely to emerge from children themselves, especially with adults becoming less hospitable hosts for infection as vaccinations rise.
Just as important, vaccinating children quickly will improve the odds of emerging from this crisis sooner. The United States is likely to need to vaccinate children to reach herd immunity, as Dr Anthony Fauci, the nation's leading infectious disease expert, and others have noted.
Clinical trials to prove that the vaccines are safe for use in children are under way. But we need to be prepared for the reality that those trials will not generate the kinds of blockbuster results that the studies of adults did.
We probably won't know how effective the vaccines are at preventing severe disease in children, for example. That's because, in order to figure out whether a vaccine is effective in kids, the main outcome of the trials needs to be common enough among children to determine whether a vaccine makes any real difference.
Fortunately, at the moment, serious disease among children is too rare for any reasonable-size trial to measure. Instead, the US vaccine trials for children (and those abroad that we are aware of) will primarily focus on safety and whether the vaccines produce an immune response.
From our perspectives as a scientist and a clinician, the trials are designed to ask the right questions: Are these vaccines safe for children? What dose produces a strong enough immune response without a high number of bothersome side effects?
The downside, though, is that the results may do little to make parents feel any urgency around vaccinating their children, because many parents already feel that their children are protected.
There are early hints that some parents might be hesitant to get their child inoculated. A new study - not yet vetted by peer review - found that parents are more reluctant to take the Covid-19 vaccine compared with non-parents and that these sentiments can mirror their intentions to vaccinate their children.
That's why a sober risk assessment is in order. The coronavirus may kill as many as one in 10,000 infected children, though some studies imply the rate is lower.
That risk is significantly higher than that of serious but treatable side effects that may be seen from vaccines. The risk of severe Covid-19 is also higher for children with underlying medical conditions.
As with any vaccine, we should prepare for the likelihood that anecdotes of children getting sick after vaccination will emerge and that the vaccine will be blamed. We cannot let that deter the vaccination effort. The Centres for Disease Control and Prevention must continue to track and share reports of health issues after vaccination, as well as the usual background rates for any condition. We should avoid taking troubling stories out of context.
Parents can rest assured that once the vaccine trials for children are complete and the data is reviewed by the Food and Drug Administration, it will be considered safe to begin vaccinating kids. Assuming that happens, we will need to hurry up and vaccinate all children, making sure we reach underserved communities. That includes children abroad because any harmful coronavirus variants that emerge elsewhere will eventually reach all of us in the US.
So far, children have mostly been spared the worst aspects of this disease. For that, we are relieved. However, we owe that to a lot of luck. From here on out, we must deliberately protect them.
Jeremy Samuel Faust is an attending physician at Brigham and Women's Hospital Department of Emergency Medicine in Boston and an instructor at Harvard Medical School. Angela L. Rasmussen is a virologist at the Centre for Global Health Science and Security at Georgetown University Medical Centre. She studies host response to infection with emerging viruses, including the coronavirus.