It’s World Immunization Week, and we have a lot of successes to celebrate. There has been an 84 percent reduction in global measles deaths since 2000, a new strategy has been rolled out to include yellow fever vaccine in routine immunization in 27 African countries, and global coverage rates for many childhood vaccines are higher than ever.

But as Booker T. Washington once said: “Success is not measured by the heights one attains, but by the obstacles one overcomes in its attainment.” As a global community, we still have many immunization obstacles to overcome because coverage rates at the global, regional and even country levels often mask the pockets of unvaccinated populations, the existence of which allow diseases to fester and keeps us all vulnerable, especially children.

Throughout Europe, coverage rates for measles vaccines are well over 80 percent, which is the proportion of the population needed to be vaccinated in order to make sure that outbreaks cannot be sustained (i.e., herd immunity). Yet, this is not the case and in the last five years, measles outbreaks have occurred in Austria, Belgium, Denmark, Germany, the Netherlands and the United Kingdom.

The European CDC reports that many of these outbreaks start by unvaccinated persons in key risk groups including travelers, certain religious groups that desire vaccine exemptions, and hard-to-reach communities that may be fearful of the government or have a language barrier. These unimmunized pockets in highly industrialized countries should motivate governments to be more sensitive to the needs of specific groups and to adapt immunization services accordingly.

Also, we must not forget that immunization programs break down along with the rest of the health system during conflicts and in humanitarian and fragile-state settings. This breakdown affects babies born after a conflict has started, who usually have weak immune systems and no protection against any vaccine-preventable disease after their mother’s antibodies wear off. It also affects adolescents, and young adults are also susceptible due to waning immunity from the inability to get needed booster shots. This is exactly what is happening in the ongoing diphtheria outbreaks in Yemen; among the Rohingya refugees in Bangladesh; and in the crumbling democracy of Venezuela. If we look at the global coverage rate of DTP, the combination vaccine that protects against diphtheria, tetanus, and pertussis, it is 86 percent.

While vaccination campaigns are underway to combat these outbreaks in Yemen and Bangladesh, the more than 100 combined deaths from a disease with one of the oldest and most effective vaccines reminds us that we must be proactive in crises and work across humanitarian and development divides to avert vaccine-preventable diseases from resurging among refugee and migrant communities.

Amid the celebrations of new policies, the colorful vaccination campaigns and the proclamations of higher coverage rates, let us remember that lurking in our midst are immunization gaps that span from the crowds at Disneyland to the markets of South Kivu. Let us not let our guard down: vaccines work—but only when enough people have been immunized.