Why it's normal for COVID-19 vaccine immunity to wane, and how booster shots can help

Vaccines are the holy grails of disease prevention. These medical marvels confer significant protection against diseases — whether a childhood infection, annual influenza or a novel coronavirus — that can easily cause undue suffering.

The polio vaccine prevents polio, a highly contagious viral childhood illness that causes nerve damage and can lead to paralysis and a permanent inability to breathe. The chickenpox vaccine prevents the itchy rash in children and its potential reappearance in adults as shingles, given that the chickenpox virus stays with people after the infection and can reactivate later in life. The COVID-19 vaccine prevents serious illness, hospitalization and death due to a virus that has killed millions and caused prolonged suffering in millions more.

Vaccines can be unpleasant, but they are incredibly safe. A prick in the arm is much better than the disease.

Vaccines, however, rarely draw out a perfect response after only one dose. Vaccines administered for childhood diseases, such as polio, measles, mumps and rubella, require boosters for the vaccine to become and remain effective. Viruses also change over time, which means re-vaccination can be necessary, as with seasonal influenza.

The COVID-19 vaccines approved by Health Canada and the World Health Organization are no exception. Recent research shows that some people need an additional dose to remain protected. Now, some health-care systems, such as in the United States, are advising everyone to receive a booster vaccine. As a doctor and COVID-19 researcher, I believe everyone who can should get a COVID-19 booster.

The evidence

Several scientific studies released in October reported on the effectiveness of the Pfizer-BioNTech COVID-19 vaccine in people six months after they had received the second jab.

I have great trust in these sources because they studied what happened for many people — roughly 5,000, five million and 3.5 million, respectively, and the articles were all published in journals known for their scientific rigour. While the authors of the last study disclosed they had conflicts of interest because Pfizer funded the study, their findings were roughly consistent with research that was not funded by the manufacturer.

All the studies showed that the COVID-19 vaccine became less effective over time, which is consistent with the dynamics of other vaccines. Adults require a tetanus vaccine every 10 years, for example.

The first study, which was a laboratory-based investigation of about 5,000 people, found that the levels of antibodies that monitor for the virus decreased steadily in the six months after receiving the second dose of the vaccine. Those that kill the virus decreased rapidly in the first three months.

Critically, the researchers found the rate of virus-neutralizing antibodies went down the fastest for men, those over 65, and those with a suppressed immune system. This data supports earlier medical advice that encouraged higher-risk individuals to get a booster.

While that may sound concerning, it is consistent with a typical immune response to a vaccine. Public health agencies are taking action now because most people have a reasonable risk of re-exposure to the virus that causes COVID-19, which can cause complications even if one is vaccinated. It’s prudent to maintain high levels of antibodies to more rapidly fight off potential infections.

While these studies solely covered the Pfizer vaccine, not the Moderna, AstraZeneca or J&J vaccines, one would expect similar observations, given that the vaccines have similar effects on the immune system.

The U.S. Centers for Disease Control and Prevention updated its guidance on Nov. 19. It advises those over 50 or living in long-term care settings to get a booster, and says that everyone over 18 may get a booster. Given that the young can infect the old, and with many young people having suffered over the pandemic, truly everyone over 18 should get the booster.

Health Canada has approved the Moderna and Pfizer-BioNTech COVID-19 vaccines as boosters for adults 18 years of age and older. The provinces are rolling out their booster programs now.

Making it through another winter of COVID-19

Anxiety about being infected with COVID-19 is still high. Despite my diligence and vaccinated status, I am concerned that I could develop COVID-19 and pass it to my elderly parents.

Illnesses like COVID-19 and the flu can be deadly for older relatives. I see this repeatedly in my COVID-19 research where I am investigating the genetic features that increase the risk of severe disease.

Fortunately, we all have advice now on protecting ourselves and our loved ones. Get another dose of the COVID-19 vaccine. Get a flu shot.

Maybe I will have a sore arm and be a bit tired for a day or two after receiving my vaccines (boosters are extremely low-risk medical procedures, just like one’s primary vaccines). If it means that I know that my family and friends are safe, the inconvenience is worth it. The peace of mind in itself is a significant boon.

There is hope for a post-pandemic world, but we must still be diligent. We owe it to ourselves and our loved ones.

Do you have a question about COVID-19 vaccines? Email us at ca-vaccination@theconversation.com and vaccine experts will answer questions in upcoming articles.

Julian Daniel Sunday Willett does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.