The US Food and Drug Administration (FDA) is looking to quickly authorize COVID-19 vaccine booster shots, but there are concerns about the safety of such approvals and newly tweaked vaccines without massive trials.

The FDA issued new guidelines Monday that trials for coronavirus booster shots aimed at emerging variants can be narrower than those for the original COVID-19 vaccines. The guidelines are intended to speed up the review process amid worries that the new variants will undermine efforts to halt the virus' spread.

While new guidance also says a tweaked COVID-19 vaccine can skip the months-long process of a randomized clinical test that would compare it with a placebo, there are some concerns if the modified vaccines will be safe enough without major trials.

Some other vaccines are regularly updated similarly. Because the influenza virus evolves rapidly from one year to the next, vaccine developers have to come up with new recipes annually.

Moreover, the newly tweaked vaccines still will need to go undergo some testing. In documents released by the FDA on Monday, it said immunizations that protect against the variants could receive clearance based on immunogenicity studies, where researchers vaccinate people and then conduct laboratory tests to measure the strength of their immune response.

Those tests, similar to what is done for annual flu shots, are far simpler than the standard efficacy studies used for clearance of the initial vaccines, which involved thousands of volunteers and took months of work, according to bloomberg.com.

The new guidelines allow companies to conduct trials with a few hundred people, which are likely to take two or three months, Peter Marks, the FDA's director for the Center for Biologics Evaluation and Research, told reporters Monday.

In the future, "modified COVID-19 vaccines may be authorized without the need for clinical studies", the FDA noted. But that isn't yet the case.

"We know the country is eager to return to a new normal, and the emergency of the virus variants raises new concerns about the performance of these products," Dr Jane Woodcock, acting FDA commissioner, said in a news release.

She said, "we want the American public to know what that we are using every tool in our toolbox to fight this pandemic, including pivoting as the virus adapts".

Several drugmakers are already working to revise their vaccines to meet the rapidly evolving strains of the coronavirus.

Moderna, Oxford/AstraZeneca and Novavax are preparing for clinical trials of vaccines tailored to the new variants or boosters. Other vaccine makers including BioNTech/Pfizer have said they could adapt their shots but haven't yet announced plans for new studies.

Johnson & Johnson, which could receive authorization for emergency use from the FDA for its one-shot vaccine as soon as this week, said it is ready to ship nearly 4 million doses of COVID-19 vaccine in the US.

The FDA's new guidelines came out as there has been increased attention and concern over new variants identified around the world. New studies reported on Tuesday that a variant first identified in California in December is more contagious than earlier forms of the coronavirus.

The studies helped explain the state's dramatic surge in COVID-19 cases and deaths. The variant discovered in California not only spreads more readily but also evades antibodies generated by COVID-19 vaccines and is associated with severe illness and death, researchers said.

"The devil is already here," said Dr Charles Chiu, a virologist at the University of California, San Francisco. "I wish it were different. But the science is the science."

The new analysis is under review by the public health departments of San Francisco County and the state and is expected to be posted late this week to MedRxiv, a website that allows new research to be shared before its formal publication.