The COVID-19 pandemic has been a long and rocky road, and two words have come to symbolize its exit ramp: herd immunity.
Since the start of the vaccine rollout, public officials have stressed that immunizing enough of the population would eventually stop the spread of the coronavirus. Each day, an online dashboard tracks San Diego Countys progress toward that goal.
But what happens once we reach it?
If youre expecting the virus to vanish, think again, says Natasha Martin, an infectious disease modeler at UC San Diego.
The biggest misconception is that something magical and immediate and drastic will happen when we hit this level of immunity, she said.
It will come with a whimper, not a bang.
Thats in part because were already seeing the benefits of vaccination. Immunizing close to the entire population against COVID-19 would help further, protecting both the vaccinated and unvaccinated. But getting to that point will be a struggle due to the rise of viral variants and issues of vaccine hesitancy and access. And were already seeing a clear slowdown in the pace of the rollout.
Martin and many others including Dr. Anthony Fauci, the nations top infectious disease expert say its even possible that well never quite reach herd immunity. But they also say that continuing to vaccinate as many people as possible, as quickly as possible will allow us to live alongside the virus safely by making infections less frequent and less severe.
In other words, the pandemic will end, but expect the coronavirus to stick around.
Vaccines protect communities by depriving a virus of the one thing it cant do without: hosts.
Viruses are essentially teeny-tiny parasites. They slip inside your cells, take them hostage and create countless viral copies that spew out of infected cells, often killing them in the process. And then they repeat that process again and again.
If enough people have been vaccinated or recovered from infection, that leaves fewer places for a virus to go, eventually breaking the cycle of transmission. And that point is known as herd immunity.
Herd immunity doesnt just help the vaccinated; it helps those who havent gotten their shots, or whove been inoculated but didnt mount a strong immune response. Thats because these people, who are still vulnerable to COVID-19, are now mostly in contact with immune individuals, who are far less likely to spread the virus.
That person who is in the herd but didnt perhaps get the vaccine or get the full protection from the vaccine, theyre also protected because the herd is protected, said Dr. Davey Smith, UCSDs chief of infectious disease research.
These arent hypothetical, far-off benefits of vaccination. Theyre already happening and have helped get us to a point where, on Monday, the county reported just 47 new cases, the lowest count since April 2020.
I do believe that the reductions in case rates that weve seen recently are occurring predominantly because of the expansion of vaccinations, Martin said.
But she cautions that we still havent immunized enough people to avoid a potential resurgence in cases if everyone (especially the unvaccinated) suddenly abandoned basic public health precautions.
Reaching herd immunity could prevent surges in a mask-free future. But how many people need to be vaccinated to get there? Martins best estimates are in the range of 85 percent to 90 percent, perhaps more.
Those figures apply to the entire population, meaning about 3 million people in San Diego County would need to be vaccinated or have recovered from COVID-19. Thats far higher than the countys goal of fully vaccinating 75 percent of San Diegans 12 and up, roughly 2.1 million people.
Martin thinks a higher target is warranted for a few reasons. One is that while children younger than 12 arent yet eligible for coronavirus vaccines, they, too, can get COVID-19. Another factor is the rise of faster-spreading viral variants, especially B.1.1.7, a strain first spotted in the U.K. and that now accounts for most new cases in the U.S. Theres evidence that this variant is 40 percent to 50 percent transmissible.
Its going to take time to reach those high levels of community-wide immunity, especially since the pace of vaccination has slowed. The number of San Diegans getting their first vaccine dose has declined for six weeks in a row, from about 123,500 a week in early April to nearly 60,000.
So far, 1.4 million residents have gotten the single-shot Johnson & Johnson vaccine or the two shots of the Moderna and Pfizer vaccines needed for full immunity against the coronavirus. At the current rate, its unlikely the county will reach 2.1 million fully vaccinated San Diegans by their goal of early July, let alone the higher figures Martin believes are needed.
We know its going to be a little bit of a grind, said Supervisor Nathan Fletcher during the countys weekly coronavirus briefing on Thursday. Were going to continue to strive to hit that goal. I think we will at some point get there. It is more an organizing tool around giving us something to strive toward.
Jaeden Johannesson, Ashley Skoglund and Emily Skoglund, from Minnesota, take a selfie near some sea lions in La Jolla on Tuesday, April 27, 2021 after the CDC said that fully vaccinated people can go outdoors without masks, except in crowded settings.
(Sandy Huffaker / For The San Diego Union-Tribune)
Regardless of when we cross that goalpost, infection rates likely wont dip dramatically. As people start getting together more, the virus will have more chances to find those who havent been vaccinated. And slow but steady vaccination offset by people resuming pre-pandemic activities will probably mean case rates will continue declining gradually, according to UCSDs Smith.
Its also likely that outbreaks will continue in places where vaccination rates are low. Infectious disease models rely on population averages, which dont tell the whole story. If 85 percent of people have been vaccinated, its unlikely the remaining 15 percent are spread evenly throughout a community. Some places will have high vaccination rates, and others wont, either because of differences in access to shots or interest in getting them.
Thats already true in San Diego County, with vaccination rates lower in East County than elsewhere in the region. Its also true worldwide, with plenty of countries that have only vaccinated a small percentage of their citizens, allowing the virus to spread largely unchecked in places like India.
An uneven vaccine rollout could eventually make coronavirus outbreaks look a bit like measles outbreaks, according to Smith. A single person carrying the measles virus can infect 12 or more people, but the spread of the virus is mostly contained through high vaccination rates. There are, however, still outbreaks in communities where immunization rates are low, such as in Marin County, New York Citys Orthodox Jewish Community and the Philippines. Occasionally, those outbreaks spill out into the wider community.
Smith and Martin say its unlikely well ever eradicate the coronavirus not any time soon, anyway. Theres only one virus scientists have wiped out with a vaccine: smallpox. The World Health Organization began that effort in 1959, declaring the disease eradicated by 1980.
Heres the good news: Eradication isnt necessary. The key is vaccinating more people as we return to normal. So far, scientists have yet to find a viral variant that no vaccine works against. Maintaining a high percentage of vaccinated people, which limits the viruss chances to mutate, will help make sure things stay that way.
We dont need to have the goal of eradication to be able to get back to a point as a society where we can live alongside that virus and go about our daily lives with minimal interruption, Martin said. You just want to be able to live alongside it so that it doesnt result in severe disease.
U-T staff writer Paul Sisson contributed reporting.
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