Subtitles section Play video Print subtitles (pensive music) - [Narrator] Throughout the pandemic, athletes have been the most tested population on the planet, sometimes providing examples of emerging theories about COVID-19. And as countries around the world continue to fight back COVID-19, attention has turned to Tokyo where the Olympics are under way. One challenge some athletes are facing: testing positive for the virus, despite having been vaccinated. This isn't the first we've heard of this. - What are chances of getting COVID-19 after receiving a vaccine? Well, you've got a better chance of buying a winning lottery ticket but it happened to Warriors player Damion Lee. - [Reporter 1] He will be out at least two weeks. It was not a false positive as first thought, even though Walman had been vaccinated. - [Reporter 2] Yankees GM Brian Cashman says that three players tested positive and three others are likely infected. About 85% of the team is vaccinated, including those who have the virus. - [Narrator] These examples are known as breakthrough infections, which occur when someone tests positive for COVID-19 at least two weeks after getting their final shot. Do these breakthrough cases mean the vaccines are failing? - No, the vaccines are not failing. The vaccines are working extremely well, and as expected. They do protect the majority of recipients from severe disease. - [Narrator] That's what vaccines are designed to do: Prevent death and severe disease. But most vaccines, including those created to fight COVID-19 don't completely protect you from infection. So it's not all that surprising that breakthrough infections are showing up. As of July 12th, more than 159 million people in the US have been fully vaccinated. CDC data suggests just under 5,500 have had breakthrough infections, resulting in hospitalizations or deaths. That's one in approximately 29,000 people who have been vaccinated. - Breakthrough infections are something that we want to monitor but in terms of their overall influence in the pandemic, they play a much smaller role than transmission among people who haven't been vaccinated. - [Narrator] But these cases raise questions about our immunity to the virus and hint at a future in which it isn't gone completely and we learn to live with it. That's due in part to variants. Research shows that variants, including Delta, can partially evade the immune response from prior infection and vaccination. - We mount a really good immune response against the virus that our body's trying to recognize. Your body is really good at recognizing and neutralizing those specific threats. But when the virus starts to change, sometimes it doesn't recognize the virus as well. And so that's how you sort of see it chip away at that immune response. That's one of the reasons that health officials are really sort of concerned about this global vaccination drive in order to prevent the virus from spreading, both to save lives and to prevent it from further mutating and evading immune response. - [Narrator] The Delta variant is the most contagious version of the virus to be identified, but research suggests that full vaccination is still protective against severe disease and death: the outcomes that have made COVID-19 so devastating. What dictates whether someone is more likely to get infected, even if they're vaccinated? Dr. Hatziioannou says there are four main variables. First is the amount of virus that is circulating in your community. - So if a great number of people around you are infected, then the possibility of you getting exposed obviously increases. If you're in close proximity with people that are infected, particularly those that are unvaccinated and have generally higher viral loads, then the probability of you getting infected increases. - [Narrator] The second is tied to vaccine uptake. - So if a large proportion of the population is vaccinated, then your virus transmission, virus loads, everything decreases. So the chances of spreading the virus amongst this population obviously decreases. - [Narrator] That's because vaccinated people act as a kind of shield, even when they do get infected. A recent CDC study found that vaccinated people carried less virus and potentially didn't spread it as much as unvaccinated people. Cases were also shorter and less severe. Vaccines help create a kind of immune memory of what a virus looks like, helping the body fight it off more quickly when it spots it. That makes it harder for the virus to spread overall. Roughly half of all Americans are fully vaccinated but in some states and globally, the vaccination rate is much lower, giving the virus more opportunity to spread and mutate. That's why having large gathering, like concerts or the Olympics can be so challenging from a public health perspective. Third, individual behavior matters. - So as measures have been abandoned, such as masking and social distancing, when you don't have a significant number of the population vaccinated, then the ability of the vaccines to protect you from getting infected decrease. - [Narrator] Finally, even after vaccination, individual immune systems vary in their ability to prevent and fight off infection. Older and immunocompromised people seem to be more susceptible to breakthrough cases, and those tend to be severe. - That's why health officials are considering additional doses right now, primarily for people who have a compromised immune system and might not have produced a good immune response after two doses of the vaccine. - [Narrator] Breakthrough cases might be asymptomatic or mild, so people may not know to get tested. But that's not the case for athletes. - We're actually seeing breakthrough infections happen a lot more amongst sports teams, like baseball or for the Olympics because those are the people that get tested pretty regularly, even if they're vaccinated. - [Narrator] At the Olympics, organizers of the games are scrambling to deal with a rising load of athletes and officials who are testing positive upon arrival in Japan, some with breakthrough infections. The vaccination rates are low in Japan. Cases there are rising and organizers didn't require those participating to get vaccinated. Dr. Hatziioannou and many other experts remain concerned that holding a large-scale international event prior to reaching a critical mass of vaccinated individuals has the potential to contribute to the virus's spread around the world. - The Olympics is a special event and it's truly remarkable that it brings all these people from all these different countries together but it also poses a perfect ground to mix variants and spread the virus that will then go back to each athlete's country.