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What is a breakthrough coronavirus case?

There are cases of fully vaccinated people being infected with covid-19 but the CDC expected this and says vaccines are still the way out of the pandemic.

Temperatures are taken from people waiting to receive their Covid-19 vaccines from AltaMed Health Services in Los Angeles, California on August 17, 2021.
Frederic J. BrownAFP

More than 60% of the total population have received their first dose of the vaccine in the US, with 169.2 million people that have been fully vaccinated. But there are concerns of the effectiveness of vaccines with the emergence of 'breakthrough' cases. These have been the result of the spread of the Delta variant which is much more infectious compared to previous variants during the pandemic. The current data suggests that the vaccine protects against most variants currently in circulation around the US but some vaccines have a reduced effectiveness when dealing with Delta.

What is a breakthrough case?

A breakthrough coronavirus case is when someone who has been fully vaccinated, and is considered immune, is infected by the virus.

Currently, it has been reported across 49 US states and territories that there have been 8,054 patients who were hospitalized from a breakthrough infection. However, the number of breakthrough cases reported to the CDC are likely to be an underestimate as the organization  partially stopped monitoring for reported breakthrough cases in May. Data also heavily relies on voluntary reporting, so that figure might not be a complete or representative picture. To date, no patterns have emerged from the data that would indicate a demographic as more vulnerable to a breakthrough case than others.

What can be done to prevent them?

Investigating the demographics of breakthrough cases could provide the first indication if a vaccine is less effective in certain groups or that vaccine-induced immunity is waning. Currently, Pfizer and Moderna have been submitting data to the FDA to have their vaccines fully-approved and to try and get a third dose out to the public by Fall.

The CDC investigated a covid-19 outbreak in a nursing facility in Kentucky, where it was reported that a high number of breakthrough cases were appearing in fully vaccinated residents. During the investigation, it was discovered that about 25% of the fully vaccinated residents were infected, but that 67% of the positive cases were asymptomatic. Therefore, it was suggested by the CDC that the vaccines are less effective against the spread of the Delta variant but they still do a good job at protecting individuals against its severity.

However, these are only preliminary findings and there is still much uncertainty about the long-term effects of covid-19. What this case does highlight is the importance of having the vaccine to minimize the risk of hospitalizations associated with the virus.

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What has the CDC said about breakthrough cases?

In their website section about breakthrough cases, the CDC notes, “Vaccine breakthrough cases are expected. Covid-19 vaccines are effective and are a critical tool to bring the pandemic under control. However, no vaccines are 100% effective at preventing illness in vaccinated people. There will be a small percentage of fully vaccinated people who still get sick, are hospitalized, or die from covid-19.”

What measures are being taken to stop the spread?

States have begun imposing vaccine mandates to prevent the spread of covid-19. As of Thursday, Governor Jay Inslee imposed a covid-19 vaccine mandate in Washington state, requiring all staff and teachers to get vaccinated, following the path of New York and California which have similar mandates.


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