Why have millions of Americans missed their second COVID-19 shot? 

According to a recent  report in the The New York Times , reasons for not getting the second dose vary, from fear of potential side effects of the inoculation, to the belief that one dose is sufficient protection against the coronavirus, to logistical barriers beyond the individual's control, such as canceled appointments and pharmacies not stocking doses of the right brand of vaccine. 

The Center for Disease Control and Prevention (CDC) data quoted in the NYT report and other media outlets has been making the rounds with headlines that emphasize that millions of Americans aren't returning for their second dose. What some of these articles are less quick to acknowledge, though, is that while 8% of Americans aren't returning for their second dose — for various reasons — that means that 92% of Americans are compliant with the dosing schedule laid out by authorities. 

Dr. Angela Rasmussen is a virologist with the Vaccine and Infectious Disease Organization (VIDO). "I thought that [CDC data] was pretty encouraging. While 8% is still millions of people, that still means that the vast majority of people are going back for their second shot, which is really good compared to a lot of other vaccines," she told DW.

Rasmussen recently experienced the double dose dilemma herself. She had an appointment for a BioNTech-Pfizer shot in Washington state, but was set to move to Canada to start a new job before she would be able to get her second dose within the recommended time frame. Instead of getting the first shot of the BioNTech-Pfizer, she got the Johnson & Johnson vaccine, a one-shot inoculation. The experience left her sympathetic to that 8% who have missed the second dose. 

"If it's this hard for me to figure out how I'm going to get a second shot, and I'm a virologist, I imagine it can be very difficult for people who don't have access to transportation, or for people who might not be as internet-savvy," she said.

Why are people skipping the second dose? 

Experts believe there are many reasons people could miss their second dose. "There are some people who may have read speculation that one shot is good enough," said Rasmussen, which could lull them into a false sense of security. Another reason that could discourage people from seeking their second dose is the post-vaccination symptoms. 

"Some people may have had unpleasant side effects related to the first shot and decided for themselves that they didn't want to experience that again with the second shot," she said. Further, a small number of people may have just forgotten they need to get a second shot.

"I think other people have been advised by their health care provider to not get a second shot, and that's if they've had an allergic reaction or have a bad history with getting vaccine boosters — that's probably a minority of cases but I think some people for medical reasons will have been advised not to get the second shot," said Rasmussen.

Demographic factors can also play a role. Dr. Lisa Cooper, head of the Johns Hopkins Center for Health Equity, said African Americans face particular structural barriers to accessing even the first dose of the vaccine. 

"If you're an older person of color you probably don't have a fancy cellphone or a computer, so you're most comfortable using a regular telephone — but some vaccination sites have no call-in number and the only way to register is to go online," she told DW. "Even if you do get through, then you find out the vaccine site is on the other side of town, and you don't have a car — do you want to get on public transit during COVID? And it's going to cost you extra money to do all of those things."

Economic concerns also play a role in vaccine hesitancy. Many US workers say they would like to get a vaccine but don't always get the support of their employers. To address this, US President Joe Biden has urged employers to provide paid leave for vaccinations and offered tax credits to small- and medium-sized businesses to cover the costs.

Is one dose enough?

Experts have repeatedly explained that in order to have long term and better protection against COVID-19, you need two shots. There's encouraging data that suggests that at least in the short term the first shot does offer some protection, but how long that protection lasts is unclear — and it's not as good as getting the second shot, as Toronto-based doctor Michael Warner pointed out recently on Twitter.

Two recent studies from the UK's Office for National Statistics and the University of Oxford found people experienced a strong antibody response after receiving either jab. One of the studies found a 72% reduction in symptomatic infections after one dose of the BioNTech-Pfizer or AstraZeneca vaccine. After two doses of BioNTech-Pfizer, though, there was a 90% reduction in symptomatic infections. Due to the timing of the rollout of the AstraZeneca vaccine, data on second doses wasn't available.

Koen Pouwels, author of the study and a researcher with the University of Oxford's Nuffield Department of Population Health, said getting your second vaccine dose leads to longer protection as well as better protection from COVID-19. 

"As with any infectious disease, initially [after the first dose] you'll have high protection, and eventually that effectiveness will wane," said Pouwels. "If you look at the antibody responses, the waning will be relatively quick. The second dose is very protective and brings your antibody levels up much higher, especially in old people," he told DW. "It's very important to get that second dose."

It's not that the second shot won't work if you wait longer before you get it; it's that you may not have the protection that you think you have during that interval between the shots. "What people may be missing is that that second shot is thought to be really needed to elicit these long-term memory immune responses," said Rasmussen. 

Is a single vaccine dose sufficient for COVID survivors?

A US study carried out by the Cedars-Sinai Medical Center involving more than 260 participants has shown that a "single dose of the Pfizer-BioNTech vaccine for individuals who previously had COVID-19 generates an immunologic response similar to that of individuals receiving the two-dose recommended sequence."

According to the researchers, the data suggests that a second dose may not be needed for those who have recovered from a coronavirus infection. The question is how effective a single dose would be.

"Many individuals have had COVID-19, and vaccine availability is still limited in most regions. So single dosing of individuals with past COVID-19 could significantly accelerate the number of people protected by immunization and help reach community levels that stop the spread of the infection," Jonathan Braun, professor of medicine at the F. Widjaja Foundation Inflammatory Bowel and Immunobiology Research Institute at Cedars-Sinai and co-author of the study, told DW via email.

Research carried out by the Penn Institute of Immunology also found that people who have recovered from an infection displayed a strong antibody response after the first mRNA vaccine. In a news release, immunologist and co-author of the findings, E. John Wherry, said that "these results are encouraging for both short- and long-term vaccine efficacy, and this adds to our understanding of the mRNA vaccine immune response through the analysis of memory B cells." Memory B cells assist in long-term immunity.

In Germany, the Standing Committee on Vaccination (Stiko) recentlyupdated its findings to reflect new data which recommends that for individuals who have had a confirmed SARS-CoV-2 infection that resulted in COVID-19, "a single vaccination should be considered no earlier than 6 months after recovery. Because of the existing immunity after having undergone infection, a very good immune response occurs due to the 1-time booster by vaccination. All licensed COVID-19 vaccines can be used for the vaccination of convalescents."