Studies show that Delta replicates more quickly and
generates more virus particles than other variants, but vaccines still protect
against serious infections.
As the United States battles a fourth surge of
COVID-19, scientists are learning much about the Delta variant that wasn’t
known when it was first reported in India in March:
it is one of the most infectious respiratory viruses known,
it causes more severe COVID-19 than
other variants, and it is more likely to evade antibodies.
Evidence of all these traits is clear. The Delta
variant has caused a sharp rise in COVID-19 cases, hospitalizations,
and deaths across
the U.S. and the rest of the
world. Driven by relaxed social distancing and mask
guidelines, poor vaccine uptake in
parts of the U.S., and lack of availability elsewhere, Delta has rapidly become
the dominant variant in
the U.S., causing more than 93 percent of new infections,
according to the Centers for Disease Control and Prevention. It has also spread
to more than 135 countries,
according to the World Health Organization.
The secret to Delta’s success is the ease with which
it spreads. The CDC estimates that
Delta can be as infectious as chicken pox and is only slightly less contagious
than measles, which is considered one of the most
transmissible viruses. Now the Delta variant is spreading like
wildfire through the South, particularly in Louisiana, which has one of the
lowest vaccination rates in the country; only 37 percent of the population is
fully vaccinated compared to 50 percent nationally. In the U.S., daily cases
are now averaging 100,000, a nine-fold jump from mid-June.
“It's surprising the extent of how infectious this
particular variant is, and how well it can then replicate in the upper
respiratory tract. Just the increased infectivity of this Delta variant has
sort of increased our concern relative to what was there for the Alpha variant,
which was increased relative to the original virus,” says Mehul Suthar, a
virologist at Emory University.
Because the Delta variant is so much more contagious
than previous variants, CDC issued new guidelines on July 27, 2021, which recommend that even after vaccination,
people should “wear a mask indoors in public if you are in an area of
substantial or high transmission.”
A vastly more transmissible virus
To track how easily an infectious disease such as
COVID-19 spreads, epidemiologists use a metric called the basic reproductive number or R0 (pronounced
“R naught”). R0 is the average number of susceptible people that each infected
person is expected to infect. It is difficult to be certain about the R0 for
ancient pandemics, but for the 1918 influenza pandemic, the average infected
person is thought to have passed the disease to between two and three people, giving it an R0 of between 2.0 and 3.0.
The first SARS coronavirus epidemic of 2002, has an R0 of three;
for the second coronavirus epidemic—Middle East Respiratory Syndrome (MERS)
first identified in 2012—R0 was between 0.69 to 1.3.
Now the CDC estimates that people infected with Delta pass the virus to between
five and 9.5 people. This is higher than
the original virus identified in Wuhan, China, which had an R0 between 2.3 and
2.7, and the Alpha variant which had an R0 between four and five. Delta can be
as infectious as chicken pox, which has an R0 between 9 and 10.
If R0 is larger than one, the number of infected
people will keep growing exponentially until all susceptible people have either
died or recovered and herd immunity is
reached. If R0 is less than one the outbreak will likely fizzle out on its own.
For the original SARS-CoV-2, herd immunity could be
reached when around 67 percent of the population was immune—either
through natural infection or vaccination. “For
Delta, those thresholds we estimate being well over 80 percent, maybe
approaching 90 percent,” Ricardo Franco, an assistant professor of medicine at
the University of Alabama at Birmingham said at a press briefing organized by the Infectious Diseases
Society of America.
A higher viral load
Delta is not only more transmissible than previous
SARS-CoV-2 variants, it can also cause more severe disease.
People infected with the Delta variant harbor about 1,000 times the number of viral particles (which
experts call the “viral load”) in their nasal swab compared to those infected
with another strain, “which is an enormous increase,” says Eric Topol, the
founder and director of the Scripps Research Translational Institute, who was
not involved in this study.
One reason for this is that the Delta variant
replicates more quickly in the nose. A study, not yet peer reviewed has
shown that the Delta variant took an average of four days to
reach detectable levels after exposure to a sick person, compared to about six
days for the original Wuhan virus.
Even after vaccination, Delta infections produced a 10-fold
higher viral load than non-Delta infections. In fact several recent
studies, none peer reviewed yet, show that vaccinated people carry the same viral load as the unvaccinated.
“We are seeing infections and seeing the large number of people being infected
by a single case, which is quite worrying. It means that the virus is highly
transmissible and is able to avoid … vaccine-induced immunity,” said Ravindra
Gupta, a clinical microbiologist at the University of Cambridge, who led the
study that is not yet peer reviewed.
Delta is also better at destroying cells because of a
mutation at position 681 of its spike protein, which is fast becoming common in other variants around the
globe and is thought to be an evolutionary game changer.
This P681R mutation makes it easier for Delta and
the related Kappa variants to invade the host cell by fusing infected cells into
structures called syncytium,
which is a way of accelerating infection. Syncytia are also formed by other viruses
such as HIV. “We found in cell culture experiments that Delta variant shows
bigger syncytia when compared to SARS-CoV-2,” explained Kei Sato, a virologist
at The University of Tokyo, Japan.
The Delta variant has also undergone multiple
mutations in its spike protein that seem to improve the virus’s ability to
bind to the ACE2 receptor and evade the immune response.
Breakthrough infections and boosters
The good news is that a complete dose of the currently
authorized COVID-19 vaccines remains effective. “All the vaccines work pretty
well," said Jeff Kwong, an infectious diseases epidemiologist at the
University of Toronto. "And the vaccines were more protective against the
severe outcomes compared to symptomatic infection,” Kwong has shown in a study, not yet peer reviewed,
the effectiveness of Pfizer, Moderna, and AstraZeneca vaccines against
symptomatic infection, hospitalization, or death between December 2020 and May 2021.
Many studies have shown that Moderna and Pfizer vaccines still protect against Delta, though not as well as against previous variants.
Unvaccinated people make up more than 90 percent of confirmed new cases among
the states that track case data along with vaccination status.
“[Vaccines] do reduce the risk of serious outcome such
as hospitalization quite substantively,” says Aziz Sheikh, a primary care
specialist at The University of Edinburgh, Scotland, UK who showed that Delta caused twice as many
hospitalizations than the Alpha variant, which caused more severe
illness than the original SARS-CoV-2. “Overall, they are working.”
The CDC estimates that COVID-19 vaccination reduces the
risk of SARS-CoV-2 infection by eight-fold and the risk of getting ill, being
hospitalized, or dying by 25-fold.
But inadequate testing nationwide makes it impossible to know the
true extent of the spread of Delta and other variants. When there is high
transmission of Delta in the community, even fully vaccinated people are vulnerable
to so-called “vaccine breakthrough infections,”
which the CDC defines as when genetic material or protein from SARS-CoV-2 is
detectable in the nasal swab more than 14 days after a person has received the
recommended dose of an FDA-authorized COVID-19 vaccine.
Two doses of the AstraZeneca and Pfizer vaccines are
estimated to be 60 and 88 percent effective, respectively, against
symptomatic disease caused by Delta.
The majority of vaccines administered in the U. S. (Moderna
and Pfizer) require two shots for maximum protection. These vaccines are much
less effective against Delta after just a single jab, says Olivier Schwartz,
head of the Virus and Immunity Unit at Institut Pasteur, Paris, who led a study
that showed that a single dose of either AstraZeneca and Pfizer vaccines
or previous infection barely inhibit Delta
variant.
Prompted by the data on the reduced efficacy against
new variants such as Delta, Pfizer is seeking authorization of a booster dose of
its vaccine. Moderna is also testing an updated mRNA vaccine booster dose. The U.S. Food and Drug Administration (FDA) is expected
to finalize a COVID-19 vaccine booster plan soon.
The single shot J&J vaccine has been shown to
be effective against the Delta variant.
But a study, not yet peer reviewed,
has shown that although all vaccines triggered the development of antibodies
that were somewhat less effective against Delta, the reduction was much steeper
for J&J than for mRNA vaccines. This study is consistent with similar ones
in monkeys and people where
two doses of the J&J vaccine show greater efficacy compared
with one dose.
To compensate for the lower efficacy of the J&J
shot against Delta, people in San Francisco who received this vaccine can now
request a “supplemental dose” of
an mRNA vaccine. Germany will begin offering mRNA vaccine booster shots in
September to a range of people considered vulnerable. However, the demand for
boosters is magnifying the inequities in COVID-19 vaccine availability between
rich and poor countries. “WHO is calling for a moratorium on boosters until at
least the end of September to enable at least 10 percent of the population of
every country to be vaccinated,” said Tedros Adhanom Ghebreyesus,
Director-General of WHO in a press briefing.
Some preliminary data from Israel suggests that efficacy of the Pfizer vaccine might decline within
six months. But this is not surprising since vaccine designers knew that making the antibody response long-lasting was
going to be a challenge. Antibodies against
the first SARS and MERS viruses declined after one to two
years. For coronaviruses that cause the common cold,
protection ranges from three to six months, and almost always less than a year.
A study in the U.S. has shown that,
following the second dose of the Moderna vaccine, neutralizing antibodies
remain high in the blood for six months. “Those antibodies that are there, for
the most part, neutralize many of these variants [including Delta]. However,
these antibody responses do wane over time,” explains Emory's Suthar, who led
the U.S. study.
Vaccines have prevented millions of infections
Vaccination may have saved
approximately 279,000 lives in
the U.S. and, by the end of June
2021, prevented up to 1.25 million hospitalizations,
according to The Commonwealth Fund’s computer models. Similarly in England the
vaccines may have prevented about 30,300 deaths, 46,300 hospitalizations, and 8.15
million infections. The aggressive
vaccination campaign in Israel is estimated to have caused a 77 percent drop in cases and a 68 percent decline in
hospitalizations from the peak of the pandemic in January, 2021.
Although COVID-19 vaccines in the U.S. are free and
effective, only 49.9 percent of the population—just
over 165 million people—are fully vaccinated as of August
4, 2021. Vaccination rates vary widely nationwide, and many
counties in southern states, including Louisiana, Florida, Arkansas,
Mississippi, and Alabama, have low vaccination rates that are now fueling
raging outbreaks of the Delta variant.
While more than 347 million COVID-19 shots have been
administered in the U.S. since distribution began on December 14, 2020, there are still about 93 million Americans age
12 and older who are eligible for a shot but have not yet received one. There
are also 48 million children under the age of 12 who are unvaccinated because
they are still not eligible. That makes it difficult to predict how long this
current surge will last.
No vaccine is 100 percent effective
With more than half the population incompletely
vaccinated, the Delta variant can continue to infect and evolve, leading to
more vaccine breakthrough infections than expected and
possibly yielding new infectious variants.
Evidence is emerging that breakthrough Delta cases may
be as transmissible as Delta infection in unvaccinated
individuals. “The vaccines are protective, but obviously a lot of
vaccinated people are also getting exposed, some from the unvaccinated and
[some] from each other. And so, it's testing the vaccine capabilities,” says
Topol.
But most breakthrough cases, which occur in less than one percent of
fully vaccinated people, cause mild
or no symptoms. Of the more than 164 million people fully vaccinated
nationwide there were only 7,525 patients with COVID-19 vaccine breakthrough
infections who were either hospitalized or died in the U.S.
through August
2, 2021.
Breakthrough infections are more likely among health care workers who
are in frequent contact with infected patients, older age vaccinated people,
and those with weakened immunity,
such as people with cancer and prior organ transplant.
Breakthrough infections are also more likely to occur in situations of close
contact, such as in large public gatherings,
restaurants, cramped working spaces,
and outdoor or indoor parties.
While vaccines can effectively slow down the
contagious pandemic by increasing the herd immunity,
preventive measures such as social distancing and masking are
proven strategies along with vaccination in curbing the spread of the virus.
“Even if people have been vaccinated, they can still get infected and can still
spread the virus in the population. So that means variants have the chance to
get more mutations or to evolve. It is important for people to stop giving the
virus [that] chance,” says Sato.
