Picture it: You get your first dose of the COVID vaccine, have your appointment for the second dose lined up a few weeks later, and then — something happens. Maybe you have to travel for an emergency, or there's a vaccine production shortage, or the pharmacy's fridge breaks. What could happen if you delay or miss that second shot of COVID vaccine?
"There are a number of potential risks to delaying or missing the second dose of any vaccine, including the Pfizer and Moderna vaccines," Dr. Bob Bollinger M.D., a professor of infections diseases at Johns Hopkins University and founding member of emocha health, a telehealth service, tells Bustle. You could be less protected against COVID, for a shorter period, or it could also give the virus time and space to mutate.
What Happens If You Miss Your Second Shot?
The prime worry about missing your second shot, Dr. Bollinger says, is that both the Pfizer and Moderna jabs have been designed to be most effective in two doses. "With two shots, both of these vaccines provide >90% protection from COVID-19," he says. The first shot teaches the immune system to recognize and fight against COVID, while the second reinforces that lesson and intensifies your body's response. After one shot, neither vaccine is at its most effective, and its effects may also wear off without the booster shot's help, though there's no real evidence to confirm this.
"People who receive only one shot or a delayed second shot may not be as well-protected from COVID-19," Dr. Bollinger says. "It is also possible that receiving only one shot or delaying the second could mean a shorter duration of protection."
Based on current data, if you have to miss a shot, it's better if it's a Moderna one. A study of the Pfizer Phase III trials published in December 2020 in New England Journal of Medicine found that the first dose only provides 52% protection. The FDA's analysis of Moderna's vaccine data from the same month found that its first dose is up to 80% protective. AstraZeneca's shot, which is not yet authorized by the FDA, is 64.1% effective after its first jab, per a study in The Lancet.
And these doses may also wear off. Pfizer and BioNTech told The BMJ in January that "there is no data to demonstrate that protection after the first dose is sustained after 21 days.”
The other worry about missing a jab, according to Science News, is that people with only one shot of the COVID vaccine might be more likely to trigger variants on the virus, like those found in the UK and South Africa. Those variants occur because the virus had time to get into peoples' cells and evolve, similar to how not finishing a course of antibiotics can lead to antibiotic resistance. This is far less likely if you've been fully vaccinated; the immune system fights the virus off before it can start mutating. Though this scenario is still theoretical, it's a concern for researchers.
What About If Your Second COVID Shot Is Delayed?
Some countries, like the U.K., are experimenting with the timing between vaccine shots, to help avoid shortages. They're prioritizing giving people their first shot, even if it means they'll have to wait up to 12 weeks for their second — and President Biden is considering a similar scheme. There's no evidence about whether Pfizer's shot is affected by a delay, but results in The Lancet show that the AstraZeneca shot was actually more effective if there were more than six weeks between doses — 65.4% effective, versus 53.4% effective for less than six weeks. Generally, however, doctors aren't keen on messing with the prescribed dose regime more than strictly necessary.
"The information we have about these vaccines come from large scale clinical trials, where the vast majority of participants took both vaccines on schedule," Dr. Lalitha Parameswaran, M.D., a clinical assistant professor in the Division of Infectious Diseases and Immunology at NYU Langone Hospital–Brooklyn, and co-lead of the vaccine research center at NYU Langone Hospital–Brooklyn, tells Bustle. "We do not have enough information on how well a single dose will protect from getting COVID."
The vaccine race could create one solution to this problem. Johnson & Johnson is currently starting Phase III trials of their vaccine, and Phase 1/2a results published in the New England Journal of Medicine in January show it could be most effective as a one-dose jab. This may also make it easier to distribute, because people won't have to go back for a booster shot a few weeks later.
Dr. Parameswaran recommends that you should attempt to get your vaccine dose on schedule if you possibly can. If you can't, though, you should be prepared to be less protected against COVID, and to act accordingly, masks and all.
Dr. Bob Bollinger M.D.
Dr. Lalitha Parameswaran M.D.
Polack, F. P., Thomas, S. J., Kitchin, N., Absalon, J., Gurtman, A., Lockhart, S., Perez, J. L., Pérez Marc, G., Moreira, E. D., Zerbini, C., Bailey, R., Swanson, K. A., Roychoudhury, S., Koury, K., Li, P., Kalina, W. V., Cooper, D., Frenck, R. W., Jr, Hammitt, L. L., Türeci, Ö., … C4591001 Clinical Trial Group (2020). Safety and Efficacy of the BNT162b2 mRNA Covid-19 Vaccine. The New England journal of medicine, 383(27), 2603–2615. https://doi.org/10.1056/NEJMoa2034577
Sadoff, J., Le Gars, M., Shukarev, G., Heerwegh, D., Truyers, C., de Groot, A. M., Stoop, J., Tete, S., Van Damme, W., Leroux-Roels, I., Berghmans, P. J., Kimmel, M., Van Damme, P., de Hoon, J., Smith, W., Stephenson, K. E., De Rosa, S. C., Cohen, K. W., McElrath, M. J., Cormier, E., … Schuitemaker, H. (2021). Interim Results of a Phase 1-2a Trial of Ad26.COV2.S Covid-19 Vaccine. The New England journal of medicine, 10.1056/NEJMoa2034201. Advance online publication. https://doi.org/10.1056/NEJMoa2034201
Voysey, M., Clemens, S., Madhi, S. A., Weckx, L. Y., Folegatti, P. M., Aley, P. K., Angus, B., Baillie, V. L., Barnabas, S. L., Bhorat, Q. E., Bibi, S., Briner, C., Cicconi, P., Collins, A. M., Colin-Jones, R., Cutland, C. L., Darton, T. C., Dheda, K., Duncan, C., Emary, K., … Oxford COVID Vaccine Trial Group (2021). Safety and efficacy of the ChAdOx1 nCoV-19 vaccine (AZD1222) against SARS-CoV-2: an interim analysis of four randomised controlled trials in Brazil, South Africa, and the UK. Lancet (London, England), 397(10269), 99–111. https://doi.org/10.1016/S0140-6736(20)32661-1