It is when a fully vaccinated person becomes infected with the coronavirus. A small number of these cases are expected and health officials say they are not cause for alarm.
Covid-19 vaccines work by teaching the body to recognize the virus. So if you are exposed to it after vaccination, your immune system should be ready to jump in and fight it.
In the studies, the two-dose Covid-19 vaccines from Pfizer and Moderna were around 95% effective in preventing disease, while the single-dose injection from Johnson & Johnson was 72% effective, although direct comparisons are difficult. So while vaccines are very good at protecting us from the virus, it is still possible to get infected with mild or no symptoms, or even get sick.
If you end up getting sick despite vaccination, experts say vaccines are very good at reducing the severity of the disease, the main reason for getting vaccinated.
Most people with breakthrough infections experience mild illness, said Dr. William moss, a vaccine expert in the Johns Hopkins Bloomberg School of Public Health.
In the US, people who were not vaccinated account for nearly all Covid-19 hospitalizations and deaths.
It is difficult to determine why a particular breakthrough case occurs. The amount of virus you are exposed to could be a factor, Moss said. Our individual immune systems will also affect the way we respond to vaccines. Some people, for example, have health problems or take medications that could make their immune system less responsive to vaccines.
People may also have been exposed to the virus before the injections took effect. Although less likely, they may have received a dose that was improperly stored or administered, Moss said.
The Centers for Disease Control and Prevention notes that variants could be factors in some important cases, although the evidence so far indicates that vaccines used in the US protect against them.
Health officials are also on the lookout for signs that breakthrough cases are increasing, which could indicate that protection against vaccines is fading and that boosters are needed.
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