The “4th booster shot” refers to the second booster dose of an mRNA COVID-19 vaccine, following the initial primary series and first booster. This shot was developed to address the changing nature of the SARS-CoV-2 virus and waning immunity. The goal was to maintain protection against the virus as it evolved.
Eligibility and Recommendations
Initial recommendations for a second booster were directed at populations considered to be at higher risk. Public health agencies, like the CDC and FDA, first made adults aged 50 and older eligible to receive a second booster. This was based on their increased risk of severe outcomes from COVID-19.
Eligibility was also extended to people who were moderately or severely immunocompromised. This included individuals undergoing cancer treatment, organ transplant recipients, and those with other conditions that weaken the immune system. The guidance for younger, healthy populations differed, recommending they stay up to date with an annual vaccine.
Purpose of Additional Boosters
The recommendation for an additional booster dose is rooted in two primary principles. The first is waning immunity, as the level of protective antibodies generated by vaccination naturally declines. Studies show that while initial doses provide strong protection, this protection can decrease in the months following vaccination, making individuals more susceptible to infection.
The second reason relates to the evolution of the SARS-CoV-2 virus. New variants emerged that possess mutations allowing them to partially evade immunity from earlier vaccines. The Omicron variant and its subvariants, for example, showed a reduced susceptibility to the antibodies generated by the original vaccines. An additional booster was intended to broaden the immune response to fight these newer forms of the virus and reduce the risk of severe illness.
Vaccine Composition and Effectiveness
The composition of the vaccines has evolved to counter new viral variants. The initial vaccines were monovalent, meaning they targeted only the original strain of the SARS-CoV-2 virus. In response to the Omicron variant, bivalent vaccines were developed. These formulations target both the original virus strain and specific Omicron subvariants, such as BA.4 and BA.5.
Data indicated that these bivalent boosters improved protection. Studies showed that receiving an updated booster dose restored protection against hospitalization. For instance, during periods when BA.2 and BA.5 were prevalent, the effectiveness of a third dose against hospitalization was high but waned after about 120 days. The updated vaccines were designed to broaden the immune response to these circulating variants, proving effective at preventing severe disease, hospitalization, and death. More recent vaccine updates for the 2024-2025 season target newer variants like KP.2 and JN.1.
Timing and Potential Side Effects
The timing for receiving an additional booster dose is an important consideration. Health officials recommended a specific interval after a person’s most recent shot. For the second booster, the guidance was often to wait at least four months after the first booster dose. For individuals who had a recent COVID-19 infection, it was suggested they could wait about three months before getting their next vaccine dose.
The side effects associated with the fourth dose were reported to be consistent with those from previous COVID-19 vaccinations. Commonly reported reactions include pain, redness, or swelling at the injection site. Some people also experienced systemic side effects like fatigue, headache, muscle pain, chills, or fever. These reactions are typically mild to moderate and resolve on their own within a few days.