Millions of people are missing their 2nd COVID-19 doses, and that has experts worried about herd immunity

• The CDC said millions of people who got one dose of a COVID-19 vaccine were missing their second.
• Some fear adverse reactions, and others are simply unable to get the second dose.
• Experts worry it could be more difficult to achieve herd immunity if that 8% increases.

Over the weekend, data from the Centers for Disease Control and Prevention showed millions of Americans who've had their first vaccine dose against the coronavirus were not receiving their second.

Many are unable to receive their second dose because of circumstances out of their control, like limited vaccine supply, scheduling issues, and access. Some are choosing to forgo their second dose because they believe they're sufficiently safe after receiving the first or they fear adverse reactions from the second.

In total, about 8% of people who received their first dose have missed their second, the CDC said.

"I don't think it's people hesitating or not wanting to get it. It's hard to get this stuff, two shots in a row," Ashish Jha, a physician at the Brown University School of Public Health, said in a Tuesday teleconference. "And so we've got to be patient, and we've got to give people more opportunities to come back and complete their second shot."

One man had difficulty scheduling a 2nd-dose appointment

Octavio Venegas told Insider his 89-year-old mother, Gladys, missed her second vaccine appointment because she was hospitalized after having a stroke and needing to treat an ulcer.

Venegas said his mother, a patient with advanced Alzheimer's, received her first Pfizer shot on March 1. She was scheduled to get her second one on March 22. A week after receiving her first dose, she had to go to the hospital for "a duodenal ulcer and she had to have two transfusions," Venegas said.

Venegas said doctors did not specify whether his mother's hospitalization was related to her vaccination dose. But her doctors were aware she had received her first shot and had not suggested to Venegas that the Pfizer vaccine could have led to her hospitalization, he said. She had also been experiencing constipation issues for a while before her stay in the hospital, which suggested to Venegas it wasn't the vaccine that caused the issues, he said.

His mother stayed in the hospital for three weeks, from March 8 to 26, missing her second vaccine appointment. After she was discharged, she went home to receive hospice care, where she remains "completely bedridden" and uses a urine catheter, Venegas said. Nurses from the hospice provider bathe and care for her.

In the past few weeks, Venegas said he had been trying to get his mother the second dose. He called the pharmacy where she received her first shot to ask about rescheduling her second. He said he also called various vaccination sites and clinics in Florida, where they live.

He asked pharmacies and clinics about the possibility of administering an at-home second dose, since his mother cannot leave her bed and can no longer use a wheelchair to get around.

All the sites he contacted either told him they could not administer vaccines outside their facilities or said they could not schedule individual doses, Venegas said. Providers, therefore, must schedule two doses for people receiving an appointment, he said he was told.

The Florida Department of Health did not respond to Insider's request for comment asking about the process to reschedule second-dose vaccine appointments.

Venegas said he had been stressed about the process, worrying about his mother contracting the virus.

"Most likely, I will have to let it go and just, in my mind, rely on the fact that only one of the two vaccines provide some level of protection," he said, adding that was "better than nothing."

"Hospice sends nurses and also people to bathe my mom in bed, and I wonder if some of these frequent and daily visitors might have had contact with people with COVID," Venegas added.

Since vaccines began rolling out in December, health officials have recommended people get two doses of either the Pfizer or Moderna vaccine to most efficiently protect against the coronavirus.

Pfizer's doses are 21 days apart, while Moderna's are 28 days apart. But those timelines are just guidelines. The CDC says people can get the two doses up to six weeks, or 42 days, apart.

"Up to 42 days, there should be no change in efficacy," Elena Cyrus, an infectious-disease epidemiologist working at the University of Central Florida, said.

Meanwhile, the Johnson & Johnson vaccine, which was un-paused on Friday and is being administered across the US, requires only one dose.

People might miss their 2nd-dose appointment because life gets in the way

For many, missing their second dose was out of their control.

The appointment time, for example, might not work for them three or four weeks after the initial dose. A lot of people don't even know what their schedule is going to be like four weeks from today.

Both Jha and Cyrus suggested the people likeliest to miss their second dose vaccines were vulnerable or marginalized groups. People from these groups might have less control over their schedules and live in areas with more access barriers and sociocultural issues.

That's why there should be more opportunities for people to reschedule their missed doses, experts say. As the US gets closer to achieving herd immunity, there will be less demand for vaccines, Cyrus said.

"There will be less emergency sites. And vaccine administration will move from federal sites to more primary care, and access to care in the US is not equitable for all communities," she added.

Structural sociocultural issues like racism and poverty mean navigating health systems can be difficult for people of color and other marginalized groups.

"This will require more synergy between clinicians, researchers, public-health practitioners, and government entities," Cyrus said.

Jha tamped down some of the overreaction around the number of people missing doses.

"People miss appointments because life happens," Jha said. "And the messaging that needs to go out right now to people is: 'Don't worry about it. Go get your second shot.'"

Some are opting out of the 2nd dose on purpose

Jo Henrion, 72, said she experienced cardiac arrest shortly after receiving the first dose and wasn't sure whether it was brought on by the Moderna vaccine.

She received her first Moderna dose on March 18. Six days later, she was hospitalized for 12 days because of the cardiac arrest, she said. Since then, doctors and specialists have been running tests and collecting blood from her.

Her second-dose appointment was April 15, but she skipped it, fearing her first dose spurred her cardiac arrest.

"I wouldn't risk another cardiac arrest for hardly anything," Henrion told Insider. She said she'd "have to really be convinced that it wasn't the shot" that caused it.

Doctors have not told her the cardiac arrest was caused by the vaccine, but Henrion said they also hadn't ruled it out. But specialists have recommended she avoid getting the second dose until they find out what caused the cardiac arrest, Henrion said. Cardiac arrest is not listed as an adverse effect by the CDC.

A home nurse tending to Henrion told her to contact Moderna to report the "adverse reaction" and ask about the cardiac arrest, she said. Henrion then got on the phone with a Moderna representative, who promised to "report it to her higher-ups and that they then will report it to the CDC," she said.

Henrion doesn't believe her cardiac arrest was brought on by any preexisting health conditions, she said.

"I'm a very active 72-year-old," she said. "So for me to be down is difficult. I don't do well laying in the bed and being not productive."

She added: "I've been a realtor for 33 years. I get a lot of exercise in my work, and I've been renovating a house for the last 12 months and a garden."

Moderna did not respond to Insider's request for comment.

By choice or circumstance, epidemiology experts say missing doses reduces the chance of reaching herd immunity

Cyrus of the University of Central Florida stressed the importance of receiving both doses of either vaccine to reach the highest immunity level possible.

"Any individual that is not fully vaccinated or those who have not received both doses for the two-dose regimen potentially do not have the full protection the vaccine offers, increasing their risk of transmission and potentially also contributing to community spread," Cyrus told Insider.

Food and Drug Administration data determined the first dose is quite effective on its own. Both the Pfizer and Moderna vaccines are likely to be 80% effective after just one dose.

The first dose of either vaccine "trains the immune system" to recognize the virus. The second dose "boosts antibody levels to afford even better protection," Dr. Francis Collins, the director of the National Institutes of Health, wrote in February.

But despite the relatively high efficacy rate of a single COVID-19 vaccine, herd immunity is still threatened by the 8% of people who have so far not received a second dose.

"If the 8% increases over time, the chances of achieving herd immunity reduces and makes the population more vulnerable to existing strains of the virus and other emerging variants or mutations," Cyrus said.

Achieving herd immunity is becoming more and more important as the US moves toward reopening, operating in pre-pandemic ways, and relaxing mask and social-distancing guidelines.

Others believe 8% is a relatively small number of people. Jha stressed that 92% of vaccinated people have received both doses.

He said it was fine if people missed their second doses because they could reschedule them.

"People should keep their second appointment, but if you miss it, you can go back and get your second shot at any point," Jha said. "Everybody absolutely needs to get their second shot."

Hilary Brueck contributed reporting.

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