Severe allergic reaction to Moderna's Covid-19 vaccine is "rare," US health authorities said Friday, with only 10 cases arising from more than four million first doses.

The Centers for Disease Control and Prevention also emphasized that widespread vaccination was critical to fighting the coronavirus pandemic.

"Based on this early monitoring, anaphylaxis after receipt of Moderna Covid-19 vaccine appears to be a rare event," the CDC said in a report, indicating that none of the reactions resulted in death.

The CDC data documented the 10 cases of anaphylactic shock among a reported 4,041,396 first doses administered between December 21 and January 10.

They were recorded among 108 possible allergic reactions, the CDC said, with six of the cases requiring hospitalization, while the other four were treated in an emergency department setting.

The numbers represent 2.5 cases of anaphylactic shock per one million Moderna injections, compared with 11.1 cases per one million Pfizer injections, according to the CDC study.

"The clinical and epidemiologic characteristics of anaphylaxis case reports after receipt of Moderna Covid-19 vaccine are similar to those reported after receipt of the Pfizer-BioNTech Covid-19 vaccine," the CDC said.

Of the 10 severe cases, all were women, aged 31 to 63, and nine had a history of prior allergic reaction, although not to vaccines.

A heavily female ratio was also noted with the Pfizer vaccine, which experts say could be due to the fact that more women than men have been vaccinated, at a rate of approximately two to one.

Symptoms began within 15 minutes after injection for nine of the Moderna shot recipients, and after 30 minutes for one.

Vaccination sites must be equipped and able to treat severe allergic reaction, and able to transport patients to the hospital if necessary, the CDC said.

"Widespread vaccination against Covid-19 with highly effective vaccines represents a critical tool in efforts to control the pandemic and save lives," the agency added.