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When someone gets the COVID-19 vaccine it triggers and immune response that will help them fight the virus that causes the deadly disease.

Dr. Karen Kaul is a pathologist with NorthShore University HealthSystem

“It is a messenger RNA vaccine,” she said. “So unlike old vaccines that inject either an inactivated virus or a piece of viral protein, what this does is inject a little snippet of viral RNA which is taken up in the muscle and into the cells to help us make antibodies.”

For years people have been taking the flu vaccine to boost an immune response against the flu virus. But when they take it in a certain form it can illicit a false positive flu test.

“The flu mist is actually inactivated virus,” Kaul said. “What we have seen with flu mist is there’s a little bit of that inactivated virus still in the nasal area or pharynx. So if one samples a patient for an influenza test too soon after they’ve received the flu mist, you can pick up the virus and detect it. It will give you a positive PCR.” 

The COVID vaccine is designed differently. It does not contain actual virus, merely a part of the spike protein in the virus – the compound that helps it invade and take hold of healthy cells.

“It’s not the whole virus. It’s not inactivated virus. But it’s just a little snippet of viral protein and that will stimulate our bodies to make an antibody response against the antigen,” Kaul said.

Kaul said she and other pathologists wondered whether the viral protein, expressed through cells in the body would trigger a similar false positive test result for COVID among vaccine recipients.

“We’ve actually been looking at that and most of the assays do not target the spike protein,” she said. “I can’t tell you all of the assays are free of that potential interaction.”

Like many things with this novel coronavirus, there are no definitive answers. But experts hope the COVID tests will be accurate and the vaccine will be effective.

Studies showed the vaccines had an efficacy of 95 percent in studies. It remains to be seen how effective they will be in real world conditions.