Maybe you have already postponed your second COVID-19 vaccine appointment, whether it is because of trouble scheduling or general reluctance. But how safe are you after only one dose?

As an immunologist, I often hear this question-as new coronavirus gene strains become more common, the answer has changed. Until early July, Delta variant Has become the most important strain of SARS-CoV-2 circulating in the United States

Moderna and Pfizer mRNA vaccines are not specifically designed to protect against Delta variants.Although in general they still Provide excellent protection New research shows that after two complete doses Single dose provides lower immunity It is more effective against the existing coronavirus strains than against the original strains.

Bottom line: Two shots are better than one shot.

How effective is the vaccine?

Soon after Pfizer’s COVID-19 vaccine was approved in December, Israeli researchers found that Single dose is very effective Among the thousands of vaccinated medical staff in a medical center. Compared with unvaccinated people, 4 weeks after injection, a single dose can reduce the infection rate by as much as 85%.

This real-world discovery is consistent with an analysis Pfizer’s clinical trial data Published in the New England Journal of Medicine in 2020. In this study, the 52% protection of the first dose of the vaccine included infections that occurred within the first 12 days after vaccination, when people would not expect the vaccine to have time to produce protective antibodies.

Another real-world study conducted by the Department of Public Health of the United Kingdom on adults aged 70 and over in early 2021 determined that A single dose of Pfizer vaccine has an effective rate of 61% Prevent symptomatic diseases 28 days after vaccination. Two doses increase the effectiveness to 85%-90%.

So, what has changed?

In essence, it boils down to a new variant.Scientists pay particular attention to the Delta variant because it Seems particularly contagious.

All COVID-19 vaccines Production of antibodies against spike glycoprotein On the surface of the coronavirus. If you encounter a coronavirus after vaccination, these antibodies will protect you by binding to the spikes on its surface, preventing the virus from entering your cells and causing infection.

The problem is that Delta variants can evade some (but not all) of the antibodies produced by current vaccines.

So far, what is the protective effect of the vaccine on Delta?

It appears that the Delta variant is relatively resistant to anti-spike antibodies produced by vaccination. This change makes it even more important to obtain a second dose of mRNA vaccine.

First shot Introduce your body To the spike protein of the virus, so your immune system can begin to produce targeted antibodies and immune cells. Second shot Give your body the opportunity to practice its immune response to COVID-19 again. The second dose will trigger the production of more anti-spike antibodies. These antibodies are more effective in protecting you because if you encounter a virus spike, they will bind more closely to the virus spike.

In a study published in the journal Nature in July, researchers tested sera in the blood of 16 recent Pfizer vaccinators in France.After the first dose of mRNA vaccine, the serum comes from Only two of the 16 vaccinated people neutralized the Delta variant Viral. However, the good news is that after the second vaccination, the sera of 15 out of 16 people neutralized the Delta variant.

Outside of the laboratory and in the real world, Public Health England has collected data on all symptomatic COVID-19 cases in the country, which has genetically sequenced the coronavirus. Of the 1,054 cases of delta infection as of mid-May, a preliminary analysis that has not been peer-reviewed found that One dose of Pfizer vaccine is 33% effective Prevent symptomatic infections. The protection rate rose to 88% after two doses. These levels of protection for Delta are lower than the older Alpha variants they found: 51% effective after the first dose and 93% after the second dose.

A small preliminary study from Canada has not yet been peer-reviewed, confirming Similar level of protectionIn 165 people with Delta infection, the researchers found that one dose of Pfizer (Pfizer) has a protection rate of 56% against symptomatic infections, and two doses are 87%. Importantly, the researchers calculated that even after a single dose, the protection rate of Delta against hospitalization or death was 78% for Pfizer and 96% for Moderna.

Am I protected?

If you have completed the vaccination, you will be well protected: two doses of Pfizer or Moderna or a single dose of Johnson & Johnson vaccine. If you have received only one of the two required doses of mRNA vaccine, then you should complete the vaccination with a second shot.This will improve your protection against COVID-19 May be as low as 33% Better than unvaccinated people Up to 90%.

If you have a weakened immune system, the situation will be more complicated. The study found, Some immunocompromised patients Does not produce antibodies After vaccination.In these cases, some studies have shown Boosters may bring hope, The third dose of mRNA vaccine triggers a protective antibody response.

For the mRNA vaccine against COVID-19, CDC recommendations remain unchanged: For Pfizer, two doses are taken 21 days apart, and for Moderna, two doses are taken 28 days apart. Sticking to a schedule and taking two doses at the same time means that once your body has time to build immunity, you will get a very high level of protection.

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Editor’s note: Since this article was originally created conversation On March 19, 2021, the coronavirus continued to mutate. This updated version reflects research as of July 2021 showing that a single dose of Moderna or Pfizer COVID-19 vaccine is not sufficient to reliably prevent infection. It is recommended to still accept the complete process of two shots.

William Petrie, Professor of Medicine, University of Virginia

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