Covid-19 vaccines work by teaching the body to recognise the virus. So if you’re exposed to it after vaccination, your immune system should be ready to spring into action and fight it but there is a chance of getting infected.
What is a Covid-19 vaccine “breakthrough” case?
It’s when a fully vaccinated person gets infected with the coronavirus. A small number of such cases are expected and health officials say they're not a cause for alarm.
Covid-19 vaccines work by teaching the body to recognise the virus. So if you’re exposed to it after vaccination, your immune system should be ready to spring into action and fight it.
In studies, the two-dose Covid-19 vaccines by Pfizer and Moderna were around 95 percent effective at preventing illness, while the one-shot Johnson & Johnson shot was 72 percent effective, though direct comparisons are difficult.
So while the vaccines are very good at protecting us from the virus, it’s still possible to get infected with mild or no symptoms, or even to get sick.
If you do end up getting sick despite vaccination, experts say the shots are very good at reducing the severity of the illness — the main reason to get vaccinated.
“When you hear about a breakthrough infection, that doesn’t necessarily mean the vaccine is failing,” Dr Anthony Fauci, the US government’s top infectious disease specialist, told a worried Senate panel this week. The shots are holding up, he said, even in the face of the highly contagious delta variant that is burning through unvaccinated communities
Most people with breakthrough infections experience mild illness, said Dr William Moss, a vaccine expert at the Johns Hopkins Bloomberg School of Public Health.
READ MORE: BioNTech shot produces 10 times more antibodies than China's Sinovac: study
Difficult to determine why they happen
In the US, people who weren't vaccinated make up nearly all hospitalisations and deaths from Covid-19.
It’s difficult to determine why any particular breakthrough case happens. How much of the virus you’re exposed to could be a factor, Moss said.
Our individual immune systems will also affect how well we respond to the shots. Some people, for example, have health conditions or take medications that could make their immune systems less responsive to the vaccines.
People might also have been exposed to the virus before the shots took full effect.
Though less likely, they may have received a dose that was improperly stored or administered, Moss said.
The Centers for Disease Control and Prevention notes variants could be factors in some breakthrough cases though evidence so far indicates the vaccines used in the US are protective against them.
Health officials are also watching for signs that breakthrough cases are rising, which could signal that protection from the vaccines is fading and boosters are needed.
READ MORE: WHO says West should recognise authorised Chinese vaccines
Are breakthrough infections symptomatic?
Reports of athletes, lawmakers and others getting the coronavirus despite vaccination may be disconcerting for vaccinated people wondering how to balance getting back to normal with more exposure to unvaccinated strangers.
One critical question about breakthrough cases is whether the person actually had symptoms, Dr Francis Collins, director of the National Institutes of Health, told The Associated Press.
"Or is this somebody just being sampled out of an abundance of caution because they had to go into some place like the Congress?” he added.
Indeed, the US Centers for Disease Control and Prevention has said for months that vaccinated people don’t even need to get tested after a virus exposure unless they develop symptoms.
But the agency cites limited evidence that they’re less likely to infect others than unvaccinated people who get an asymptomatic infection.
Different places have different rules. In Britain, Prime Minister Boris Johnson – who survived Covid-19 early in the pandemic and now is fully vaccinated – began quarantining over the weekend after contact with someone experiencing mild symptoms from a breakthrough infection.
Breakthroughs tend to be mild because a vaccinated person's immune system doesn't have to start from scratch to fight the coronavirus.
Even if the virus sneaks past vaccine-spurred antibodies and starts replicating in your nose or throat, secondary defenses jump into action and usually, “the virus is stopped in its tracks within a few days,” said University of Pennsylvania immunologist Scott Hensley.
While there's not a specific count, it's clear breakthrough infections are rare. As of July 12, the CDC had tallied 5,492 vaccinated people who were hospitalized or died and also tested positive for coronavirus — out of more than 159 million fully vaccinated Americans.
CDC Director Dr Rochelle Walensky has said 99.5 percent of all deaths from Covid-19 are in the unvaccinated.
There isn't a separate count of mild or asymptomatic breakthroughs, although CDC is tracking those through studies such as one that gives weekly virus tests to more than 5,000 essential workers, she told senators.