With Covid-19 shots reaching billions of people, reports have grown more common of people getting infected with the coronavirus despite being vaccinated. Just as a natural infection doesn’t guarantee protection from reinfection with the virus, neither does immunization provide a perfect shield. Still, those who have immunity -- either from vaccination or infection -- carry a fraction of the risk of those who have none. So-called breakthrough cases among the immunized are a reminder that as long as the pandemic virus is prevalent in the world, it remains a threat to everyone.
First, it’s important to remember that testing positive indicates an infection with SARS-CoV-2, the coronavirus that can cause Covid-19. The disease is diagnosed only when the infection causes symptoms such as fever and cough; a significant proportion of people who become infected never develop symptoms. Although vaccines provide a strong defense against severe illness caused by SARS-CoV-2, none fully protects against the infection, meaning many vaccinated people are still at risk of catching the virus and of transmitting it to other people. The more SARS-CoV-2 is circulating in a community, the higher the chance of infection. In some instances, those infections will break through the protective shield that vaccine-induced immunity provides to cause Covid symptoms. In rare cases, the illness may be life-threatening. Another possible risk is so-called long Covid -- lasting fatigue, breathlessness and other symptoms seen in an estimated 1 in 10 Covid survivors; it’s unknown how well vaccines prevent these lingering problems.