Bill and Melinda Gates warned in their annual letter Wednesday that the lasting legacy of the coronavirus pandemic could be "immunity inequality" — a wide and deadly gap between wealthy people, with easy access to coronavirus vaccines, and everyone else.

Why it matters: As long as there are large swaths of the world that can't get vaccinated, they warned, it will be impossible to get the pandemic under control.

"From the beginning of the pandemic, we have urged wealthy nations to remember that COVID‐19 anywhere is a threat everywhere," Melinda Gates said in the letter.

• "Until vaccines reach everyone, new clusters of disease will keep popping up. Those clusters will grow and spread. Schools and offices will shut down again. The cycle of inequality will continue."
• "Everything depends on whether the world comes together to ensure that the lifesaving science developed in 2020 saves as many lives as possible in 2021."

Bill Gates said the world should prepare better for the next pandemic by spending "tens of billions of dollars per year" — mostly contributed by wealthy countries — to improve the scientific tools for fighting infectious diseases.

• He also called for the creation of a "global alert system" to detect disease outbreaks as soon as they happen, as well as the use of "germ games" to help train first responders.
• "To prevent the hardship of this last year from happening again, pandemic preparedness must be taken as seriously as we take the threat of war," he wrote.

The backstory: Melinda Gates said in an interview with "Axios on HBO" last year that the coronavirus wiped out global gains in education, poverty eradication, vaccinations, and maternal and child health in a matter of weeks.

The bottom line: The couple said they were "optimistic that the end of the beginning is near" — and that new tests, treatments and vaccines "will soon begin bending the curve in a big way."