Why do some COVID vaccines require two doses, while others require only one?

Johnson & Johnson single shot doses of COVID-19 vaccine; why do other vaccines require two doses
Photo via New York National Guard/Flickr (CC BY-ND 2.0)

In February, the U.S. Food and Drug Administration (FDA) issued an emergency use authorization for the Johnson & Johnson COVID-19 vaccine. This made it the third vaccine to be approved in the country, along with the Pfizer and Moderna vaccines. But unlike the Pfizer and Moderna versions, both of which require two shots, the Johnson & Johnson vaccine only requires a single dose. What is the difference between these vaccine types, and how does dosage level affect the vaccine’s effectiveness and immunity offered from COVID-19?

Types of COVID-19 vaccines

The three coronavirus vaccines that are currently approved by the FDA fall under one of two main types. The mRNA vaccine, according to the CDC, is made of mRNA or “Messenger RNA”, a genetic material that instructs the body on protein production. It does not contain any part of a live virus, so it cannot give you COVID-19. 

In a nutshell, the mRNA in the vaccine teaches your cells how to make a copy of the spike protein present on the surface of the virus that causes COVID-19. It then triggers an immune response inside the body, causing the cell to break down genetic material from the vaccine. Think of it as a practice round for your body’s immune system. If you ever contract the virus in the future, your body’s production of antibodies against it will be much more effective.

The Pfizer and Moderna mRNA vaccines require two shots, spaced 21 and 28 days apart, respectively. 

The other type of coronavirus vaccine available in the U.S. is the viral vector vaccine. It contains a modified version of a completely different virus. Inside the shell of this virus, material from the virus that causes COVID-19, also known as a “viral vector,” is contained. This viral vector, once inside the body’s cells, helps to recreate the spike protein present in COVID-19 for a practice round. It then triggers antibody production and activates other immune cells to defend against what it thinks is an infection. 

Like the mRNA vaccine, the viral vector vaccine does not contain any part of a live virus and cannot give you COVID-19. It also cannot make you sick from the virus that was used as the vector.

The Johnson & Johnson vaccine is a viral vector vaccine and requires only a single dose.

Why does the mRNA vaccine require two shots?

The first dose of a two-shot vaccine sets up the groundwork for the body’s defense against COVID-19, and a second dose fortifies it even further. The time in between doses allows the body to produce an immune response gradually.

A lot of vaccines require two shots, including the MMR and shingles vaccines. Based on clinical trial results, the Pfizer and Moderna vaccines were found to be 95% effective and 94.1% effective respectively in preventing COVID-19 in people without signs of previous infection.

According to researchers, some immunity forms following the first dose. A study published in the New England Journal of Medicine found that Pfizer’s vaccine efficacy against COVID-19 was 52% after the first dose but grew to 91% within seven days after the second. 

A U.K. study yielded even more promising results, with vaccine effectiveness of 89%-91% within 15 days following the first dose. A similar report submitted to the FDA tracking the Moderna vaccine found that vaccine effectiveness was at 50.8% up to 14 days after the dose was administered. Effectiveness increased to 92% beyond the initial 14 days.

As noted by the FDA, however, participants of the study who did not receive the required two vaccine shots were generally only tracked for a short duration. Nothing definitive can yet be concluded about the long-term quality of protection provided just by one dose of a two-shot vaccine.

Is the Johnson & Johnson COVID-19 vaccine less effective?

According to clinical trials, the Johnson & Johnson vaccine is 66% effective at preventing COVID-19 in people without signs of previous infection. The highest percentage of protection was logged at two weeks following the shot’s administration. The vaccine was also found to be highly effective at preventing hospitalization and death in people who did get sick, and people who contracted COVID-19 at least four weeks after receiving the vaccine did not need to be hospitalized. Early evidence also shows the vaccine may provide protection against asymptomatic infection, according to the CDC.

The Johnson & Johnson vaccine yielded a lower percentage in overall effectiveness compared to the approved mRNA vaccines, but it still reportedly exceeds the 50% efficacy requirement set by the FDA for COVID-19 vaccines. It also was found to be 85% effective against severe disease in all regions studied. 

Notably, while clinical trials for the Pfizer and Moderna vaccines examined its effects on symptomatic COVID-19 infections, including mild cases, Johnson & Johnson’s vaccine research reportedly focused more on protection levels against moderate or severe cases of COVID-19.

Reminders for people who are scheduled to receive a two-dose COVID-19 vaccine

There is currently no evidence to suggest that people who only get one dose of an mRNA vaccine have adequate long-term protection against COVID-19 infection. The FDA strongly recommends that people follow the approved dosing schedule.

The CDC does not favor one vaccine brand over the other. The agency does note, however, that the vaccines are not interchangeable. The safety and effectiveness of mixing doses from multiple products have yet to be evaluated. To ensure that people stick to only one vaccine brand and complete both doses appropriately, the CDC is encouraging individuals to keep and ensure the safety of their vaccine card, which will be issued to them after getting their first shot.

The CDC also advised people to take a photo of the vaccine card with their phone as a safety measure. Vaccine recipients are additionally encouraged to sign up for apps and messaging platforms that issue second-dose reminders, register for an immunization information system account for your respective state, and nail down an appointment for the second dose before leaving the initial vaccination appointment.

Read more on the coronavirus vaccine:

Sources: CDC [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11], JAMA Network, The New England Journal of Medicine, Public Health England, FDA [1, 2], Healthline

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