In the early days of the coronavirus vaccine rollout, scientists were hopeful it might herald the long-awaited turning point of the pandemic, not only bringing the threat of severe disease, hospitalization and death to an end, but also completely halting the spread of the disease.
Dr. Anthony Fauci, chief medical adviser to the White House, said in December 2020 that if the nation's COVID-19 vaccine campaign went well, the U.S. could approach herd immunity by summer's end and "normality that is close to where we were before" by the end of 2021.
A series of events last summer, including a widespread COVID-19 outbreak that hit highly vaccinated Provincetown, Massachusetts, soon swayed those hopes, as evidence emerged that vaccinated people were contracting the virus more frequently than initially expected and transmitting it to others.
In the months that followed the Provincetown outbreak, breakthrough infections would shift from a statistical anomaly to a regular occurrence.
Experts say current vaccines are still doing their most important job - dramatically reducing people's risk of severe illness and death. But they are no longer hopeful vaccines will stop the virus in its tracks, now that it's clear vaccinated people can develop mild disease and transmit illness to others.
The astoundingly high levels of protection against infection that was initially observed, especially for the mRNA vaccines, created by Pfizer and Moderna, have largely dissipated, especially for those with one or two doses combined with extremely transmissible variants.
"When it comes to vaccines and COVID-19 infection, there's good news and bad news," said Dr. Shira Doron, an infectious disease physician and hospital epidemiologist at Tufts Medical Center. "The good news is the vaccines are still doing an amazing job at preventing serious illness, hospitalization and death. The bad news is that effectiveness at preventing infection is a lot lower in the omicron era, and wanes quickly after vaccination."
Despite the increase in the number of breakthrough infections, per capita data shows that unvaccinated Americans continue to have a greater risk of developing severe disease or death from COVID-19, than their vaccinated counterparts.
"Variant evolution and even subvariant evolution within omicron have shown that accumulation of mutations result in limited cross-protection when it comes to infection risk," said John Brownstein, Ph.D., an epidemiologist at Boston Children's Hospital and an ABC News contributor. "The main question continues to be does infection matter in a world where serious outcomes are averted."
But health experts say that despite mild outcomes for many vaccinated and boosted people, waning immunity and easily transmissible variants, high caseloads have the potential to strain public health systems and put the vulnerable at risk. In addition, long COVID-19 remains a risk, although vaccinated people are less likely than unvaccinated to suffer from it.
How protected are we really?
Although the vaccine's protection against hospitalization remains strong, data collected in the U.K. found that being vaccinated was no longer enough to protect against mild or asymptomatic infection by June 2022.
Although effectiveness against severe disease remains relatively strong across all vaccines, emerging evidence suggests that that protection also wanes over time, and in the face of newly evolved variants. A CDC analysis found three shots was roughly 90% effective against emergency room or hospital visits in the months after the third shot, but that declined to 66-78% by four to five months out.
Given mounting evidence of the benefits of three doses, the CDC recommends an additional booster dose for adults and children 5 and older.
A recent study published in the New England Journal of Medicine also found that as the virus continues to evolve, each new omicron subvariant is increasingly likely to lead to reinfection or breakthrough infection.
Researchers found lower antibody responses against new omicron subvariants BA.5 and BA.5 - now dominant - compared to prior omicron subvariants.
Thus, as evolving variants continue to escape protection, Doron stressed that widespread protection against infection is likely unrealistic.
"I believe it is not reasonable to expect the current vaccines to prevent infection," Doron said. "The effectiveness isn't high, and it is short-lived. It is not feasible to revaccinate people as often as would be needed to maintain any kind of level of protection from infection."
Amid concerns of a renewed COVID-19 resurgence, federal officials are now in the process of deciding what type of vaccines should be made available in the fall, in order to better address variants that are increasingly getting better at eluding the immune response triggered by the vaccines.
The Food and Drug Administration said this week that it is asking the vaccine companies to produce shots for this fall that will give Americans the broadest and strongest protection against COVID-19 for its upcoming booster campaign.
The manufacturers have been advised to create vaccines that include two strains of COVID-19, the original strain and the most recent omicron BA.4 and BA.5 strains that are currently dominant in the U.S.
Although the FDA's group of advisers voted earlier in the week in support of moving forward with the new vaccine design, some reminded Americans that these new-and-improved shots will also not be able to offer total protection from mild infections.
Although scientists say they are not giving up on finding a vaccine that can prevent all infections, they creating such a vaccine is more difficult - a feat that very few other vaccines have accomplished.
Herd immunity no longer the goal
In the first months of the pandemic, reaching herd immunity was frequently discussed by public health experts as an important long-term goal in achieving national protection against COVID-19 and returning to normalcy in the face of a deadly and mysterious disease.
However, when the occurrence of breakthrough infections became more common, despite mass vaccination, the likelihood of herd immunity began to slip away.
"The more our understanding of COVID-19 has improved, the more we've realized that the theoretical end state of herd immunity is unachievable," Brownstein said. "There will likely be a continuous race between the evolution of the virus and the background immunity achieved either through infection or vaccination. Unfortunately, evolution continues to outpace our ability to gain protection."
Doron said that she believes that "eventually" the nation will have enough immunity to treat COVID-19 like other viruses that circulate regularly, when hospitalizations and deaths are not as widespread.
"I think we will have enough immunity as a population to treat COVID-19 like we do other respiratory viruses, where we stay home until we feel better and don't structure our lives around the virus. Immunocompromised people will still need to be careful as they always are about catching infections," Doron explained.
Providing clear goals and outlining the benefits of vaccination remains critical to encouraging uptake among the hesitant.
"Public health has to be more clear about the goals of vaccination programs," Brownstein said.
Many health experts agree that the goal of the nation's current vaccine drive cannot be to prevent every infection, as providing robust protection from hospitalization and deaths remains the most critical goal in the nation's fight against COVID-19.
"If COVID-19 had always caused only the kind of infection that we currently see in boosted people, we wouldn't have let it upend our lives the way we did. Ultimately, what's most important is prevention of hospitalization and death," Doron said.
However, Brownstein noted that it is important to consider the continued risks of long-COVID, which even with less severe forms of infection.
"While prevention of hospitalization and deaths remain the priority, post-acute conditions like long COVID-19 challenge this paradigm," Brownstein added. "Muddied messaging has led to extreme divides between those who feel protected and those who still worry about long term impacts of infection especially with unknown variants around the corner."