As Covid-19 brought nearly every corner of the Earth to a halt early last year, researchers around the world scrambled to develop a vaccine to fend off the deadly respiratory coronavirus. And just several months later — in a process that normally takes years — several vaccines were ready for worldwide distribution. In comparison, about 40 years since the earliest reports of what became known as AIDS, scientists are still scratching their heads to develop a vaccine against the virus that causes the life-threatening disease — HIV.