RN Janelle Roper, left, administers the Pfizer Covid-19 vaccine to Nurse Anesthetist Kate-Alden Hartman.
  • The Covid-19 vaccines are highly effective, but there's still a small chance of infection post-shot.
  • Two new studies summed up a handful of "breakthrough" cases in hospital systems.
  • Other research found people who got Covid-19 after vaccination had lower amounts of the virus.
  • For more stories visit Business Insider.

We know the Covid-19 vaccines work extremely well, but we never thought they would be perfect.

Even with efficacy rates as high as 95% in Pfizer trials, there's still a small chance of getting infected with the coronavirus after you're fully vaccinated. Now that the vaccine rollout is well underway in the US, we're starting to see a few "breakthrough" cases pop up.

Two letters to the editor published in the New England Journal of Medicine this week summed up these rare post-vaccine infections at three different hospital systems.

Just four out of 8,121 fully vaccinated employees at the University of Texas Southwestern Medical Center in Dallas tested positive for Covid-19 between December 15, 2020, and January 28, 2021.

At UC San Diego Health and the David Geffen School of Medicine at the University of California, Los Angeles, only seven out of 14,990 healthcare workers tested positive two or more weeks after receiving a second dose of either the Pfizer-BioNTech or Moderna vaccines.

These findings shouldn't deter anyone from getting the vaccines; if anything, they just prove even vaccinated people need to continue taking some precautions for now, one study author said.

"We felt really strongly that this data should not lead people to say, 'Let's all get vaccinated and then we can all stop wearing masks,'" Francesca Torriani, infectious disease doctor who led the California study, told Denise Grady for the New York Times. "These measures have to continue until a larger segment of the population is vaccinated."

Even in cases of infection, vaccinated people had low amounts of virus

In any case of Covid-19, a person's viral load - the amount of virus in their system - determines how likely they are to infect another person.

Luckily, even when the vaccines have failed to prevent infection, the shots seem to keep viral loads low.

A February study from Israel, which has yet to be peer-reviewed, looked at more than 1,000 people who'd tested positive for the virus after getting both doses of the Pfizer vaccine. Those who got Covid-19 12-28 days after their second dose had four times less virus in their bodies compared to a regular case.

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