Why Does AZ Hurt More After the First Dose, While Pfizer Hurts More After the Second?

The first dose vaccination rate for COVID-19 in Korea is expected to surpass 60% within this week. [Image source=Yonhap News]

[Asia Economy Reporter Hwang Sumi] As the domestic COVID-19 vaccination completion rate reaches about 35%, interest in adverse reactions such as fever, muscle pain, and headache after vaccination is increasing.

In particular, the vaccination authorities' analysis and the experiences of vaccine recipients indicate that the degree of adverse reactions varies depending on the vaccine type and the number of doses administered.

The currently used two-dose vaccines are AstraZeneca (Oxford University), Pfizer (BioNTech), and Moderna vaccines. Among these, AstraZeneca mainly reports adverse reactions such as fever and headache after the first dose, whereas Pfizer and Moderna report more adverse reactions after the second dose.

This is analyzed to be due to different immune responses in the body depending on the vaccine type.

According to the COVID-19 Vaccination Response Promotion Team as of the 29th of last month, out of 42,048,131 COVID-19 vaccinations, 171,159 adverse reaction reports were filed, with a reporting rate of 0.41%.

The reporting rates by vaccine are AstraZeneca 0.5% (first dose 0.72%, second dose 0.12%), Pfizer 0.31% (first dose 0.27%, second dose 0.37%), Moderna 0.51% (first dose 0.47%, second dose 1.98%), and Janssen 0.65%.

AstraZeneca and Janssen vaccines use chimpanzee adenovirus and human adenovirus as vectors, respectively. These vaccines induce the body to produce antibodies through the COVID-19 spike protein gene inserted into the adenovirus.

Adenoviruses are generally known to cause epidemic respiratory diseases in children. When used as vaccine vectors, their pathogenicity is removed so that respiratory diseases are not induced. However, it is analyzed that those who have not been previously exposed to adenoviruses may experience symptoms such as muscle pain and headache.

For AstraZeneca's second dose, unlike the first dose, the body remembers the adenovirus, so adverse reactions are less likely to appear. If adverse reactions occur after the second dose, it is analyzed to be due to an increased immune response.

As of midnight on the 29th of last month, the adverse reaction reporting rate by AstraZeneca vaccination dose was 0.72% for the first dose and 0.12% for the second dose, with higher reporting rates after the first dose across all age groups. Among them, after the first dose, the rates were 3.33% for ages 18?29, 0.92% for ages 30?49, 0.66% for ages 50?74, and 0.17% for ages 75 and above. In contrast, after the second dose, the reporting rates were lower: 0.34% for ages 18?29, 0.28% for ages 30?49, 0.11% for ages 50?74, and 0.09% for ages 75 and above.

Pfizer COVID-19 vaccine officially approved by the FDA
[Image source=AFP Yonhap News]

Pfizer and Moderna vaccines are mRNA vaccines. They deliver mRNA encapsulated in lipid nanoparticles, which allows the body to produce antigen proteins and induce an immune response. The lipid nanoparticles, composed of small fat particles, help the unstable mRNA to be easily delivered into cells.

mRNA vaccines have higher adverse reaction reporting rates after the second dose than after the first. For Pfizer, the adverse reaction reporting rates were 0.27% after the first dose and 0.37% after the second dose, with the second dose being relatively higher. Moderna showed 0.47% after the first dose and 1.98% after the second dose, showing a significant difference.

The higher adverse reaction reporting rate after the second dose of mRNA vaccines is explained by a surge in additional immune responses.

While the first dose injects the 'blueprint' mRNA into the body, the second dose adds to the antibodies produced in the body according to the blueprint, resulting in more antibodies after the second dose and thus more adverse reactions.

Comparing the adverse reaction rates after the second dose of Pfizer and Moderna, Pfizer's rate is 0.37%, and Moderna's is 1.98%, showing a large difference. Some speculate that Moderna, which injects the raw solution as is, causes a stronger immune response. Pfizer dilutes 0.45 cc of raw solution with 1.8 cc of saline and injects 0.3 cc per person, whereas Moderna injects 0.5 cc of raw solution.

However, it is also pointed out that the difference in adverse reaction rates cannot be explained by dosage alone. In Korea, elderly people, who tend to have fewer adverse reactions such as fever and muscle pain after vaccination, mostly received Pfizer, while Moderna was administered more to relatively younger people, which may explain the higher adverse reaction rate.

Domestic Pfizer recipients include medical staff at COVID-19 treatment hospitals, those aged 75 and above, residents of elderly facilities, high school seniors, high school staff, university entrance exam candidates, essential workers under 30, and military enlistees, mixing various age groups except those aged 60?74 targeted for AstraZeneca. On the other hand, Moderna recipients are known to be mostly younger people, such as new hospital staff under 30 and some in their 50s.

Citizens receiving vaccinations [Image source=Yonhap News]

Meanwhile, the domestic first-dose COVID-19 vaccination rate is expected to surpass 60% within this week.

According to the COVID-19 Vaccination Response Promotion Team on the 5th, as of midnight that day, the first-dose vaccination rate was 58.4%. Out of the total population of 51.34 million, 29.99 million people have received at least one vaccine dose.

Among them, 17.74 million people have completed all recommended doses by vaccine type, resulting in a vaccination completion rate of 34.6%.

The government's goal is to raise the first-dose vaccination rate to 70% before this Chuseok holiday and to increase the completion rate to 70% by the end of next month. To achieve a completion rate above 70%, over 90% of the elderly and over 80% of adults must participate in vaccination. Vaccination rates for those in their 50s, 60s, and 70s have already exceeded 90%, and the 18?49 age group is expected to reach the mid-80% range before the target date.

This content was produced with the assistance of AI translation services.