Studies show these two vaccines for COVID-19 can effectively reduce risk by up to 80 percent

The Covid-19 vaccine developed by Pfizer Inc with BioNTech SE and Moderna Inc reduces the risk of infection by 80 percent two weeks or more after the first injection of two shots, according to data from a US real-world study released on Monday (March 29).

The risk of infection fell 90 percent within two weeks of the second injection, a study of nearly 4,000 US health care personnel and first responders found. These results validate previous research indicating vaccines start working immediately after the first dose, and ensure that they also prevent asymptomatic infections.

Several countries dealing with limited vaccine supplies have postponed schedules for a second dose in hopes of gaining protection for more people. US public health officials, however, continue to recommend two doses to be given on a schedule endorsed by regulators based on clinical trials.

A study by the US Centers for Disease Control and Prevention (CDC) evaluated the vaccine's ability to protect against infections, including infections that cause no symptoms. Previous clinical trials by the company evaluated the efficacy of their vaccine in preventing disease from Covid-19, but those studies would have missed an asymptomatic infection.

Findings from real-world use of this messenger RNA (mRNA) vaccine also confirm the demonstrated efficacy in large controlled clinical trials conducted before they received emergency use authorization from the US Food and Drug Administration.

The study looked at the effectiveness of the mRNA vaccine among 3,950 participants in six states over a 13-week period from December 14, 2020 to March 13, 2021. About 74 percent had at least one injection, and tests are performed weekly to detect asymptomatic infections.

"The official Covid-19 mRNA vaccine provides initial and substantial real-world protection against infection for our country's health care personnel, first responders and other critical frontline workers," said CDC Director Rochelle Walensky in a statement.

The new mRNA technology is a synthetic form of a natural chemical messenger used to instruct cells to make proteins that reflect parts of the new coronavirus. It teaches the immune system to recognize and attack the real virus.

The CDC study came weeks after real-world data from Israel showed that the Pfizer / BioNTech vaccine was 94 percent effective in preventing asymptomatic infections. The UK and Canada are among the countries allowing an extended gap between doses by up to three or four months. British authorities said in January that data supported his decision to take a 12-week gap between doses.

Pfizer and its partners in Germany have warned they have no evidence to prove this. In their landmark trial, there was a gap of three weeks between injections of Pfizer and four weeks for the Moderna vaccine.

The CDC said Monday's results provide assurance that people begin to develop protection from the vaccine two weeks after their first dose, although the agency reiterated that the greatest protection was seen among those who had received both recommended doses of the vaccine.