The more boosters the better? Data from Israel show that immune protection in elderly people is strengthened even further after a fourth dose. Karl Lauterbach, MD, German minister of health, recently pleaded for a second booster for those aged 18 years and older, and he pushed for a European Union–wide recommendation. He has not been able to implement this yet.

Just as before, Germany’s Standing Committee on Vaccination (STIKO) is only recommending the second booster for people aged 70 years and older, the European Medicines Agency (EMA) is recommending the fourth vaccination for everyone aged 80 years and older, and the United States has set the general age limit at 50 years.

Specialists remain skeptical about expanding the availability of the second booster. “From an immunologic perspective, people under the age of 70 with a healthy immune system do not need this fourth vaccination,” said Christiane Falk, PhD, head of the Institute for Transplantation Immunology of the Hannover Medical School (Germany) and member of the German Federal Government COVID Expert Panel, at a Science Media Center press briefing.

After the second vaccination, young healthy people are sufficiently protected against a severe course of the disease. Dr. Falk sees the STIKO recommendation as feasible, since it can be worked with. People in nursing facilities or those with additional underlying conditions would be considered for a fourth vaccination, explained Dr. Falk.

Complete protection unrealistic

Achieving complete protection against infection through multiple boosters is not realistic, said Christoph Neumann-Haefelin, MD, head of the Working Group for Translational Virus Immunology at the Clinic for Internal Medicine II, University Hospital Freiburg, Germany. Therefore, this should not be pursued when discussing boosters. “The aim of the booster vaccination should be to protect different groups of people against severe courses of the disease,” said Dr. Neumann-Haefelin.

Neutralizing antibodies that are only present in high concentrations for a few weeks after infection or vaccination are sometimes able to prevent the infection on their own. The immunologic memory of B cells and T cells, which ensures long-lasting protection against severe courses of the disease, is at a high level after two doses, and a third dose increases the protection more.

While people with a weak immune system need significantly more vaccinations in a shorter period to receive the same protection, too many booster vaccinations against SARS-CoV-2 are not sensible for young healthy people.

Immune saturation effect

A recent study in macaques showed that an adjusted Omicron booster did not lead to higher antibody titers, compared with a usual booster. In January 2022, the EMA warned against frequent consecutive boosters that may no longer produce the desired immune response.

If someone receives a booster too early, a saturation effect can occur, warned Andreas Radbruch, PhD, scientific director of the German Rheumatism Research Center Berlin. “We know this from lots of experimental studies but also from lots of other vaccinations. For example, you cannot be vaccinated against tetanus twice at 3- or 4-week intervals. Nothing at all will happen the second time,” explained Dr. Radbruch.

If the same antigen is applied again and again at the same dose, the immune system is made so active that the antigen is directly intercepted and cannot have any new effect on the immune system. This mechanism has been known for a long time, said Dr. Radbruch.

‘Original antigenic sin’

Premature boosting could even be a handicap in the competition between immune response and virus, said Dr. Radbruch. This is due to the principle of “original antigenic sin.” If the immune system has already come into contact with a virus, contact with a new virus variant will cause it to form antibodies predominantly against those epitopes that were already present in the original virus. As a result of this, too many boosters can weaken protection against different variants.

“We have not actually observed this with SARS-CoV-2, however,” said Dr. Radbruch. “Immunity is always extremely broad. With a double or triple vaccination, all previously existing variants are covered by an affinity-matured immune system.”

Dr. Neumann-Haefelin confirmed this and added that all virus mutations, including Omicron, have different epitopes that affect the antibody response, but the T-cell response does not differ.

Dr. Radbruch said that the vaccine protection probably lasts for decades. Following an infection or vaccination, the antibody concentration in the bone marrow is similar to that achieved after a measles or tetanus vaccination. “The vaccination is already extremely efficient. You have protection at the same magnitude as for other infectious diseases or vaccinations, which is expected to last decades,” said Dr. Radbruch.

He clarified that the decrease in antibodies after vaccination and infection is normal and does not indicate a drop in protection. “Quantity and quality must not be confused here. There is simply less mass, but the grade of remaining antibody increases.”

In the competition around the virus antigens (referred to as affinity maturation), antibodies develop that bind 10 to 100 times better and are particularly protective against the virus. The immune system is thereby sustainably effective.

For whom and when?

Since the immune response is age dependent, it makes more sense to administer an additional booster to elderly people than to young people. Also included in this group, however, are people whose immune system still does not provide the same level of protection after the second or even third vaccination as that of younger, healthy people.

Dr. Radbruch noted that 4% of people older than 70 years exhibited autoantibodies against interferons. The effects are huge. “That is 20% of patients in an intensive care unit – and they all have a very poor prognosis,” said Dr. Radbruch. These people are extremely threatened by the virus. Multiple vaccinations are sensible for them.

Even people with a weak immune response benefit from multiple vaccinations, confirmed Dr. Neumann-Haefelin. “We are not seeing the antibody responses here that we see in young people with healthy immune systems until the third or fourth vaccination sometimes.”

Although for young healthy people, it is particularly important to ensure a sufficient period between vaccinations so that the affinity maturation is not impaired, those with a weak immune response can be vaccinated again as soon as after 3 months.

The “optimum minimum period of time” for people with healthy immune systems is 6 months, according to Dr. Neumann-Haefelin. “This is true for everyone in whom a proper response is expected.” The vaccine protection probably lasts significantly longer, and therefore, frequent boosting may not be necessary in the future, he said. The time separation also applies for medical personnel, for whom the Robert Koch Institute also recommends a second booster.

A version of this article first appeared on Medscape.com.

Second COVID booster: Who should receive it and when?