Why don’t COVID-19 vaccinations last longer?
Measles vaccines are good for life, chickenpox vaccines protect for 10 to 20 years, and tetanus vaccines last a decade or more. But U.S. officials are wondering whether to allow COVID-19 boosters for vaccinated adults as early as six months after the initial inoculation.
The goal of a vaccine is to provide the protection offered by natural infection, but without the risk of serious illness or death.
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“A really good vaccine makes sure that someone doesn’t get infected even if they’re exposed to the virus,” said Rustom Antia, a professor of biology at Emory University who studies immune responses. “But not all vaccines are ideal. “
The three levels of defense, he said, include full protection against infection and transmission; protection against serious disease and transmission; or critical illness protection only.
Efficacy depends on the magnitude of the immune response elicited by a vaccine, the rate at which the resulting antibodies disintegrate, the tendency of the virus or bacteria to mutate, and the location of the infection.
The threshold of protection is the level of immunity sufficient not to get sick. For each bug it’s different, and even the way it’s determined varies.
“Basically it’s the levels of antibodies or neutralizing antibodies per milliliter of blood,” said Mark Slifka, professor at Oregon Health & Science University.
(T cells also help protect, but antibodies are easier to measure.)
A threshold of 0.01 international units per milliliter was confirmed for tetanus in 1942 when a pair of German researchers intentionally exposed themselves to the toxin to test the results of previous animal studies.
“One of them gave himself two lethal doses of tetanus in his thigh and watched how well it went,” Dr Slifka said. “His co-author took three lethal doses. “
Neither got sick.
A threshold for measles was set in 1985 after a college dormitory was exposed to the disease shortly after a blood drive. The researchers checked the antibody concentrations in the donated blood of the students and identified 0.02 international units per milliliter as the level needed to prevent infection.
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With these diseases, the magnitude of the response to vaccines combined with the rates of antibody breakdown to produce long-lasting immune responses: Measles antibodies break down slowly. Tetanus antibodies break down faster, but the vaccine causes the body to produce far more than it needs, offsetting the decline.
“We are lucky with tetanus, diphtheria, measles and vaccinia,” said Dr Slifka. “We have identified what the protection threshold is. You track the decline in antibodies over time, and if you know the threshold of protection, you can calculate the durability of the protection. With COVID, we don’t know. “
Historically, the most effective vaccines have used replication virus, which essentially elicit lifelong immunity.
Measles and chickenpox vaccines use replicating viruses.
Non-replication vaccines and protein-based vaccines (such as tetanus) last less, but their effectiveness can be improved with the addition of an adjuvant—a substance that increases the magnitude of the response.
The vaccines against tetanus and hepatitis A use an adjuvant.
The Johnson & Johnson and AstraZeneca COVID-19 vaccines use a non-replicating adenovirus and do not contain adjuvants. Pfizer and Moderna COVID-19 messenger RNA vaccines, that work differently, do not contain any viruses.
To further complicate matters, viruses and bacteria that mutate to evade the body’s immune response are more difficult to control.
Measles, mumps, rubella and chickenpox hardly mutate, but at least eight variants of SARS-CoV-2, the virus that causes COVID-19, have been found, according to the British Medical Journal.
“This complicates the effectiveness of the vaccine,” said Dr Slifka. “You chase multiple targets over time. The flu also mutates. With the flu, we have adapted by making a new flu vaccine every year that matches the new strain of flu as closely as possible. “
Flu shots can provide protection for at least six months.
Aside from the complexities of designing an effective vaccine to fight a shape-changing virus, some hopes revolved around whether or not COVID-19 could be beaten by obtaining herd immunity, but, according to Dr. Antia, how which coronaviruses infect the body makes that difficult.
“Vaccines are very unlikely to lead to long-lasting herd immunity for many respiratory infections,” said Dr Antia. “Collective immunity only lasts for a modest period of time. It depends on how fast the virus is changing. It depends on how quickly immunity wanes. “
Part of the problem is that coronaviruses replicate in the upper and lower respiratory tract.
“We have good circulation in our lungs and our body, but not on the surface of our nostrils,” said Dr Slifka. “We can block serious disease because there are antibodies in the lower respiratory tract. “
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But the risk of mild infections in the upper respiratory tract may persist.
In the future, COVID-19 vaccines will be updated to combat variants of the virus, and researchers at Imperial College London say the next generation of vaccines may also focus on boosting immunity. in moist surfaces of the nose and lungs.
In the meantime, avoiding the slippery virus might require another injection.