The death of former U.S. Secretary of State Colin Powell on Monday from complications of COVID-19 has provided fuel for vaccine skeptics and opponents, who immediately seized on the news that Powell had been vaccinated to stoke doubts about the effectiveness of vaccines.

But Powell’s immune system had quite likely been weakened by multiple myeloma, a cancer of white blood cells. Both the disease and the treatment can make people more susceptible to infections.

His age, 84, may also have increased his risk, scientists have said.