A breakthrough is usually associated with something positive, but with COVID-19, vaccine makers and the Centers for Disease Control and Prevention (CDC) are tracking breakthrough cases. That's when someone who is fully vaccinated still gets the coronavirus.

Some states like South Carolina have already reported dozens of suspected breakthrough cases.

There are several scenarios where this can happen.

“There's about a 5% or less chance that you actually don't respond to the vaccine and don't create antibodies, don't create immunity,” said Dr. Luis Ostrosky at McGovern Medical School.

Infectious disease doctors say the vaccines aren't 100% effective. People with compromised immune systems or older people may not create immunity.

Another scenario is more about timing. The person can become infected just before they got a vaccine, in-between doses, or before they develop full immunity, which is about two weeks after a second dose. In that case, depending on if a person needs treatment, they may have to wait a few weeks, up to 90 days, to get a second dose.

A third scenario is the most concerning. A vaccinated person may be infected by a COVID-19 variant.

The good news is, so far, breakthrough cases are rare, and symptoms are far less severe if any.

“The data from Israel is perhaps the most telling where they've vaccinated a large amount of the population already and they're seeing performance that looks pretty much like they did in the clinical trials, so very encouraging,” said Dr. Ostrosky.

The quarantine recommendations stand for breakthrough COVID-19 cases. Should someone need treatment after vaccination, they can still get anti-bodies through plasma.

Ironically, vaccinated people may not be able to donate plasma specifically for COVID-19 treatments, because they don't know yet if it's as effective as plasma from someone who actually got infected.

General plasma donations are OK and still in high demand.