Booster shots of COVID-19 vaccine probably won't cause worse side effects than the original vaccines, experts predict

• COVID-19 boosters are expected later this year.
• It's too early to say for sure what side effects to expect.
• But experts predict they won't be much worse than the original COVID-19 vaccines.

COVID-19 vaccine booster shots probably won't cause worse side effects than the original vaccines, two leading experts have told Insider - although others say it's too early to say.

A booster shot is an additional dose of a vaccine that augments the body's immune response. The tetanus vaccine requires an additional dose 10 years later to maintain immunity, for example.

A booster for COVID-19 would either be a third dose of the same two-dose vaccine, or a bespoke dose designed for variants.

Moderna, Pfizer, and AstraZeneca are all testing both options with trial results expected later this year, including to see whether an additional dose helps protect against coronavirus variants, particularly the worrisome variant from South Africa, called B.1.351, that can sometimes escape the immune response.

The original two-dose COVID-19 vaccines made by the three drugmakers usually cause side effects that self-resolve after a few days.

"It will be no different or worse than the second dose," Amesh Adalja, assistant professor at the Johns Hopkins University Center for Health Security told Insider. Adalja said that people may experience a sore arm or aches and pains.

Already, we're starting to get early data from Moderna to suggest a third dose boosts the antibody response and side effects were generally similar to the second dose.

Read more: How coronavirus variants called 'escape mutants' threaten to undo all our progress

The vaccine made specifically for the variant will use the same capsule, called a lipid nanoparticle, to deliver the vaccine's instructions to the body - it's just the instructions inside that change.

Kathleen Neuzil, professor in vaccinology at the University of Maryland, told Insider that she anticipated that the nature of the side effects would be similar because the booster vaccines use the same lipid nanoparticle.

The unknown is whether an additional third dose would cause worse side effects, she said.

Adalja said that he didn't expect that the immune system would "overreact" to a third COVID-19 dose.

He explained that tetanus boosters don't usually cause worse or different side effects to the original vaccine.

"No-one has ever called me about the tetanus vaccine boosters side effects and we give them all the time," Adalja said.

Too early to say

Insider previously reported that a participant in the Moderna trial said he experienced some arm pain the day he got the booster shot, as well as a low-grade fever, nausea, aches, and fatigue lasting about 24 hours.

Moderna said in a press release Wednesday that the side effects of the booster shots they had tested were "generally comparable" to those observed after a second dose of their vaccine.

About 11% of the people who got the variant-specific booster reported a grade-three side effects severe enough to affect daily living, compared with 15% who got the third dose of the original shot, they said.

The most common side effects were fatigue, headache, muscle pain, and joint pain.

But not many people have received a third COVID-19 vaccine dose and we still haven't got data from AstraZeneca or Pfizer.

"There aren't enough people who have had a third shot to give us an indication of what we might expect," Dr. Susan Bailey, elected president of the American Medical Association and immunology and allergy specialist told Insider.

Professor Gregory Poland, consultant and the department of molecular pharmacology and experimental therapeutics at the Mayo Clinic, said that there was a demonstrable increased immune reaction to the second dose of COVID-19 vaccines from Moderna, Pfizer and AstraZeneca.

"Until you get full data, you don't know," he said.

Boosters every 12 months

We don't yet know when COVID-19 boosters should be given or if they'll definitely be required.

Studies have indicated that two-dose vaccine protection probably lasts at least 6 months, but beyond this is unclear.

Stephane Bancel, CEO of Moderna, and Albert Bourla, the CEO of Pfizer, have both suggested a booster shot would likely be needed within 12 months of getting vaccinated to maintain protection.