AThe emergence of the Omicron SARS-CoV-2 variant has confirmed the need for vaccine boosters — to combat waning immunity and provide more robust protection against severe disease and death.
Several studiesTrusted Source have shown that two doses of a COVID-19 vaccine do not provide enough protection against infection or severe disease from Omicron.
More and more evidence supports a three-step approach, which might involve infection-plus-vaccination hybrid immunity or triple immunization. Meanwhile, a growing number of countries are now exploring second boosters — or fourth doses — as a way of effectively neutralizing Omicron.
Following in the footsteps of Israel, countries including Chile, Spain, Denmark, and Sweden are offering fourth doses to certain at-risk groups.
In the United Kingdom, groups deemed most at risk are advised to have a second booster shot this spring. Depending on the person, this could be a fourth or fifth dose.
Variations in guidance have raised questions about who needs a fourth dose and what the most effective approach to immunity might be.
What data say about the 4th dose
Data about the need for a fourth dose, so far, is lacking, but recently published research indicates that it can restore waning immunity.
A trial with the Pfizer and Moderna mRNA vaccines found that a fourth dose provided a modest boost in protection against Omicron and restored antibodies to levels seen right after the third dose. However, this did not translate into significantly increased effectiveness and did not prevent infection with Omicron.
Four doses of the Pfizer vaccine were 30% more protective against infection than three doses. This extra protection was 11% for Moderna’s vaccine.
Dr. William Haseltine, a scientist, public health expert, and president of ACCESS Health International, said initial studies showed that the fourth dose provided an additional boost in antibody levels. While not huge, this boost was “measurable.”
The additional dose could also reignite antibody production in the body when this begins to wane. According to a report by the U.K. Health Security Agency, the booster dose’s capacity to protect against symptomatic infection drops to 45–50% from 10 weeks onward.
