As the number of Americans ready for their second Covid-19 vaccine shot grows, some are falling through the cracks of an increasingly complex web of providers and appointment systems, according to Reuters.

While many people are getting the required second doses, the process is taking a toll on some of the most vulnerable — older adults who in many cases rely on family members or friends to navigate complex sign-up systems and inconvenient locations.

Available vaccines need to be given as two separate doses weeks apart, and confusion is further taxing an already challenged system. For example, Houston’s health department on Friday told those seeking a second dose to be patient, saying the volume of calls was creating long wait times at its call centre.