Can you skip your second dose of the COVID-19 vaccine?
The experts explain why that might undermine medicine’s effectiveness
You dread the prospect of aches and pains. That trip to the clinic is inconvenient and takes time. And that first dose already offers some protection.
Tempted to skip that second dose of your COVID-19 vaccine? Don’t do it, experts say. Here’s why.
Q Why do we need two doses?
A A single dose of Pfizer’s or Moderna’s vaccine was 80% effective in preventing infections, according to the CDC’s March 29 Morbidity and Mortality Weekly Report. The vaccines’ effectiveness jumped to 90% two weeks after the second dose.
The second dose is an insurance policy. In the short term, one dose is probably going to be fine. The reason for a second dose is that you’ll have a more durable response.
Q What is the second dose doing that improves upon the first?
A
The purpose of the second dose is to train your immune system to respond to this virus more vigorously.
I’ll draw a very simple analogy. If you wish to become an expert in whatever field — let’s say, playing the piano — it’s not enough just to have one lesson. If you have many lessons, you become much more of an expert.
So if you wish your immune system to become an expert at protecting you from COVID-19 infection, then you want to provide the immune system with multiple training sessions. You can get vaccinated once, but that’s not enough.
— Bali Pulendran, Ph.D., professor of pathology and of microbiology and immunology at Stanford
Q
If I skip my second dose, how long am I protected?
A
We know that those people who are fully vaccinated are protected for at least six months — and likely at least nine months.
So if you don’t get the second dose your immunity may wear off sooner, making you more vulnerable to infections. You may not get that six to nine months, or even longer, protection. It may be for a shorter period, and then that will increase the risk of a “breakthrough” infection.
— Dean Blumberg, M.D., professor and chief of pediatric infectious diseases at UC Davis Children’s Hospital
Q
What, specifically, happens to the body after that second dose?
A
There’s an enormous increase in the quantity of neutralizing antibodies. If you look at the amount of neutralizing antibodies in a drop of blood of a person who has just received a single dose, it might be — I’m just pulling a number — “5.” Once you get the second dose, it could be as much as “500” or “5000.”
That’s because the immune system has been trained and is much more of an expert at producing neutralizing antibodies against this virus.
— Bali Pulendran
Q So why is the J&J vaccine just one dose?
A The J&J vaccine went into Phase 3 trials as a one-dose vaccine because earlier phase trials had shown that was enough to trigger a strong immune response.
After one dose, across all populations, even in older people, the antibody response and T-cell response were excellent.
But its efficacy in preventing mild to moderate disease is 72% — lower than the 94-95% reported for the mRNA vaccines. (All vaccines are equally effective in preventing severe illness and death.)
What would happen if we were to administer the J&J vaccine twice? The company is now conducting a trial to give people two doses to see if that boosts its efficacy.
— Monica Gandhi, M.D., professor of medicine, UCSF — Bali Pulendran
Q
Why is there different timing for the second doses for the Pfizer and Moderna vaccines? Pfizer says to wait three weeks; the other is four weeks. A
The difference in timing is not all that much. It’s just a week. It is very likely to do with the way in which the Phase 3 trials were performed.
— Bali Pulendran
Q
If I can’t make it to my second dose on time, do I have to start over? A No. The intervals were based on the clinical trials. They’re not absolutely rigid, set in stone. Of course, one should try and adhere as much as possible to the intervals, as recommended by the FDA. But from an immunological perspective, if there’s a slight delay of one to two weeks, I would be very surprised if that had a substantial impact on the benefits.
There is still a clear benefit. Better late than never.
— Bali Pulendran Q I had COVID-19, so why must I bother with getting both doses of a vaccine? A Being infected with the virus does stimulate your immune system, and it does provide some protection.
But It’s not clear how long the protection will last. And it’s not clear how effective that protection will be. It’s not a controlled study. One person might have been infected by just a few viral particles. Another person might have had a much greater dose.
We don’t yet know the impact of these differences on the strength and duration of immunity. It’s not the same as injecting someone with a defined dose of the vaccine at a defined time point.
Immunity from natural infection just isn’t as good.
— Bali Pulendran Q With all the variants of the virus circulating, will two shots be enough? A Vaccine makers may have a financial incentive for this answer. But, Pfizer CEO Albert Bourla said in a CNBC interview Thursday, a likely scenario is that there will be a need for a third dose, somewhere between six and 12 months. It is also possible that COVID-19 vaccinations will be annual, just like flu shots.