SAN FRANCISCO (KPIX) — Thousands of doses of the Johnson & Johnson vaccine were pulled over concerns about a rare but serious side effect. As Bay Area counties hit pause on distributing the vaccine, doctors are learning more about how effective they are overall, and for how long.

In the Bay Area, about 20,000 doses of the Johnson & Johnson vaccine have been shelved. Despite reports of blood clots linked with the J&J shots, doctors say vaccines are still the best way to fight COVID-19 and they may provide immunity for years to come.

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“We don’t know for sure but I think the duration of the vaccine is going to be a very long time,” said Dr. Monica Gandhi, an infectious disease doctors with UCSF.

Dr. Gandhi explains there is evidence that these vaccines could last anywhere from 10 years to a lifetime. According to Dr. Gandhi, studies show patients still have immunity to SARS, 17 years after the outbreak. What’s even more promising, in biopsies done in patients infected by the flu in 1918, they were still able to produce antibodies 90 years later.

Dr. Gandhi adds, “If we look at past coronavirus that cause severe illness, not the ones that cause colds, I truly believe we are not going to need frequent boosters.”

The Centers for Disease Control though is looking at the Johnson & Johnson vaccine to see if there is any link to 6 rare cases of blood clots in women out of nearly 7 million doses administered in the United States.

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Dr. Yvonne Maldonado from Stanford University is on the advisory committee that will explore if the vaccine played a role.

“What other effects may have been associated with the vaccine, what other characteristics of these individuals we might know about and whether or not to change the recommendations for the Johnson & Johnson vaccine,” says Dr. Maldonado.

While the CDC investigates, doctors want to emphasize severe side effects are extremely rare and urge people to continue getting vaccinated.

“Basically at this point the best thing to do is to get the vaccines out, which are really safe and really effective to as many people as possible to get through this pandemic and to save lives,” says Dr. Gandhi.

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The advisory committee will meet Wednesday afternoon. One of the recommendations it may make is to only have the Johnson & Johnson vaccine available to men or post menopausal women.