1 Shot, 2 Shots, 3 Shots, But Why?
Studies of COVID-19 vaccines have shown the need for a booster shot to improve immune responses to the variants of COVID-19. Recently, a study showed Pfizer's Comiarty Covid-19 vaccine had a 6 month efficacy period. So much so, in many countries, inoculations for third doses or boosters are already on the way.
- The primary goal of any vaccine is to provide protection to its host against natural infections but without the risk of serious illness or death.
- No one vaccine or virus is the same. The threshold of protection is the level of immunity that is sufficient to keep us from getting sick. Every virus is different, and even how it is determined varies.
For instance, Pfizer & Sinovac has been shown to be up to 10 times more effective if a booster is given after 6 months. Moreover, reports suggest that vaccines ultimately may become a yearly ordeal. So why is it that jabs protecting against measles and chickenpox have a significantly greater lifespan than Covid-19 vaccines?
Remember taking your measles shot (most likely, when you were a child), if you did, well you are protected for life. For chickenpox, immunizations last up to 20 years. With tetanus, hep B, etc. most shots last anywhere from 5 to 10 years.
The primary goal of any vaccine is to provide protection to its host against natural infections but without the risk of serious illness or death.
Rustom Antia, a biologist at Emory University denotes that “a really good vaccine makes it so someone does not get infected even if they are exposed to the virus." Unfortunately, “not all vaccines are ideal."
Vaccines provide three tiers of defense against infections:
- The Best: Full protection against infection and transmission;
- The Important: Protection against serious illness and transmission; or
- The Minimum: Protection against serious illness only.
The effectiveness of a vaccine is subject to whether the virus or bacteria tend to mutate, the magnitude of the immune response a vaccine induces, how fast the resulting antibodies decay, and the location of the infection.
However, no one vaccine or virus is the same. The threshold of protection is the level of immunity that is sufficient to keep us from getting sick. Every virus is different, and even how it is determined varies.
In its essence, vaccines' effectiveness is based on “levels of antibodies or neutralizing antibodies per milliliter of blood," said Dr Mark Slifka, a professor at Oregon Health & Science University.
(T helper cells 2 (Th2) also contribute to the protection, but antibodies are easier to measure.)
In measuring tetanus, a threshold of 0.01 international units per milliliter was confirmed when a pair of German researchers intentionally exposed themselves to the toxin to test the findings of previous animal studies.
Dr Slifka explains “one of the German researchers gave himself two lethal doses of tetanus in his thigh and monitored how well it went, and the other did three lethal doses."
The experiment was a success as neither scientist got sick.
For measles, the threshold was identified after a college dorm was exposed to the disease shortly after a blood drive. The threshold level of 0.02 international units per milliliter as the level needed to prevent infection was found after checking antibody concentrations in the students' blood donations.
With tetanus and measles, the magnitudes of response to the vaccines combined with the antibodies’ rates of decay to produce durable immune responses. While measles antibodies decay slowly, tetanus antibodies decay more quickly. However, the vaccine causes the body to produce far more than it needs, offsetting the decline.
“We’re fortunate with tetanus, diphtheria, measles, and vaccinia," Dr. Slifka said. “We have identified what the threshold of protection is. You track antibody decline over time, and if you know the threshold of protection, you can calculate the durability of protection. With COVID, we don’t know.
Historically, the most effective vaccines used replicating viruses, which can elicit lifelong immunity. Measles and chickenpox vaccines use replicating viruses.
Non-replicating vaccines and protein-based vaccines (such as the one for tetanus) won't last as long. However, their effectiveness can be enhanced with the addition of an adjuvant — a substance that enhances the magnitude of the immune response. In the vaccine for tetanus and hepatitis A, these vaccines use an adjuvant.
COVID-19 and Its Effectiveness
The Astrazeneca & Johnson & Johnson COVID-19 vaccines use non-replicating adenovirus and do not contain an adjuvant. The Pfizer and Moderna messenger RNA COVID-19 vaccines, which work differently, don’t contain any virus at all. Sinovac is inactivated virus, which also is non-replicating.
Compounding this, virus mutations that can escape the body’s immune response are harder to control much like COVID-19 and its variants.
In comparison to COVID-19, measles, mumps, rubella, and chickenpox rarely mutate. However, according to the British Medical Journal, at least eight variants of SARS-CoV-2 have been found since January 2020.
“It does make it more complicated for the vaccine to work," Dr. Slifka said. “You’re chasing multiple targets over time. Flu also mutates. With flu, we’ve adjusted by making a new flu vaccine each year that as closely as possible matches the new strain of flu."
Similar to most COVID-19 vaccines, the flu vaccines can offer protection for at least six months.
Setting aside the complexities of crafting an effective vaccine to combat a shape-shifting virus, the common hope across the world has revolved around the possibility of defeating COVID-19 by achieving 'herd immunity'; however, according to Dr. Antia, the way coronaviruses infect the body makes that challenging.
“Vaccines are very unlikely to lead to long-lasting herd immunity for many respiratory infections," Dr. Antia said. “The herd immunity only lasts for a modest period of time. It depends on how fast the virus changes. It depends on how fast the immunity wanes."
Part of the problem is how the virus replicates compounded by where the virus replicates in its host. SAR-CoV-2 replicates in both the upper and lower respiratory tracts.
“We have good circulation in our lungs and body, but not on the surfaces of our nostrils," Dr. Slifka said. “We can block severe disease because there are antibodies in the lower respiratory tract."
But the risk of low-level infections in the upper respiratory tract can persist.
Moving forward, COVID-19 vaccines will be updated to combat variants of the virus, and according to researchers at Imperial College London, the next generation of vaccines might also focus on enhancing immunity in the moist surfaces of the nose and lungs.
So, in the meantime, our best option to stay ahead of COVID-19 may require us to get another shot, which can be seen in many countries, including Malaysia, opting for a booster or third dose.
This article is an adaptation of the Wall Street Journal article made for Healtiyer readers.