Even when researchers have an answer, Offit added that he would be surprised if a SARS-CoV-2 vaccine is ever much more than 70% effective at stopping infections. That’s because of the short incubation time that respiratory illnesses, like the flu or COVID-19, give even a prepared immune system to ramp up defenses and release antibodies. It takes four to five days for people exposed to the coronavirus to begin to develop symptoms. In contrast, measles has an incubation time of 10 to 12 days, part of what makes the measles vaccine 97% effective for a lifetime after just two doses.

Fauci also worries that people will need somewhat frequent coronavirus vaccine booster shots, noting that immune system antibodies tend to wane after an infection with the four everyday coronaviruses that cause around 20% of common colds. Hotez was less concerned about this, arguing the immune system should still retain a memory of spike proteins, through other immune cells that can last in the body for decades. But he acknowledged that it still needs to be studied, given that SARS-CoV-2 has only been known to scientists for six months.