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COVID-19 may never go away, but practical herd immunity is within reach

It is unlikely that we will reach full herd immunity for COVID-19. However, we are likely to reach a practical kind of herd immunity through vaccination.

The level of immunity needed — either through vaccination or infection — for practical herd immunity is uncertain, but may be quite high. (Shutterstock)

When people say that we won’t reach “herd immunity” to COVID-19, they are usually referring to an ideal of “full” population immunity: when so many people are immune that, most of the time, there is no community transmission.

With full herd immunity, most people will never be exposed to the virus. Even those who are not vaccinated are protected, because an introduction is so unlikely to reach them: it will sputter out, because so many others are immune — as is the case now with diseases like polio and mumps.

The fraction of the population that needs to be immune in order for the population to have “full” herd immunity depends on the transmissibility of the virus in the population, and on the control measures in place.

It is unlikely we’ll reach full herd immunity for COVID-19.

For one thing, it appears that immunity to COVID-19 acquired either by vaccination or infection wanes over time. In addition, SARS-CoV-2 will continue to evolve. Over time, variants that can infect people with immunity (even if this only results in mild disease) will have a selective advantage, just as until now selection has mainly favoured variants with higher transmission potential.

Electron micrograph of a yellow virus particle with green spikes, against a blue background.
The B.1.1.7 variant of the SARS-CoV-2 virus. Over time, variants of concern will likely continue to emerge. NIAID, CC BY

Also, our population is a composition of different communities, workplaces and environments. In some of these, transmission risk might be high enough and/or immunity low enough to allow larger outbreaks to occur, even if overall in the population we have high vaccination and low transmission.

Finally, SARS-CoV-2 can infect other animals. This means that other animal populations may act as a “reservoir,” allowing the virus to be reintroduced to the human population.

Practical herd immunity

Nonetheless, we are likely to reach a practical kind of herd immunity through vaccination. In practical herd immunity, we can reopen to near-normal levels of activity without needing widespread distancing or lockdowns. This would be a profound change from the situation we have been in for the past 18 months.

Practical herd immunity does not mean that we never see any COVID-19. It will likely be with us, just at low enough levels that we will not need to have widespread distancing measures in place to protect the health-care system.


Read more: COVID-19 variants FAQ: How did the U.K., South Africa and Brazil variants emerge? Are they more contagious? How does a virus mutate? Could there be a super-variant that evades vaccines?


What level of immunity (either through vaccination or infection) we need for practical herd immunity is uncertain, but it may be quite high. The original strain of SARS-CoV-2 was highly transmissible and transmission is thought to be higher still for some variants of concern.

Empty vials of Pfizer's COVID-19 vaccine
To achieve two-thirds immunity, 90 per cent of the eligible population would need to be vaccinated or infected naturally. (AP Photo/John Locher)

The amount of immunity we need will also depend on what level of controls we are willing to maintain indefinitely. Continued masking, contact tracing, symptomatic and asymptomatic testing and outbreak control measures will mean we will require less immunity than we would without these in place.

Some estimates suggest that we may need two thirds of the population to be protected either by successful vaccination or natural infection. If 90 per cent of the population is eligible for vaccination, and vaccines are 85 per cent effective against infection, we can obtain this two thirds with about 90 per cent of the eligible population being vaccinated or infected naturally.

The United Kingdom has already exceeded these rates in some age groups. Higher rates are even better, because there is still uncertainty about the level of transmissibility and vaccine efficacy against infection (although research shows they are very good against severe disease). We don’t want to discover that we do not have enough immunity through vaccination and have another serious wave of infection.

Emerging variants

A sticker reading 'I'm COVID-19 vaccinated' from Vancouver Coastal Health
Booster vaccinations will hopefully allow us to maintain long-term practical herd immunity against future variants of COVID-19. THE CANADIAN PRESS/Jonathan Hayward

Higher vaccine uptake will mean there are fewer infections before we reach practical herd immunity. The remaining unvaccinated individuals will be safer, protected indirectly by the immunity of those around them. Outbreaks will be smaller and rarer, and there will be fewer opportunities for vaccine escape variants to arise and spread.

That said, variants of SARS-CoV-2 will continue to emerge, and selection will favour variants that escape our immunity. Vaccine developers will continue to broaden the spectrum of the vaccines that are available, and boosters will hopefully allow us to maintain long-term practical herd immunity.

It’s possible that an immune escape variant will emerge that is severe enough, and transmissible enough, that it will cause a new pandemic for which we do not have even practical herd immunity. But barring that, while we may not be free of COVID-19, we can be confident that in the not-too-distant future it will be manageable when we return to near-normal life.

Caroline Colijn's research group receives funding from the Natural Sciences and Engineering Research Council of Canada, Genome British Columbia, the Michael Smith Foundation for Health Research, the Public Health Agency of Canada and Canada 150 Research Chair program of the Federal Government of Canada.

Paul Tupper's research group receives funding from the Natural Sciences and Engineering Research Council of Canada.

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There will soon be one million seats on this popular Amtrak route

“More people are taking the train than ever before,” says Amtrak’s Executive Vice President.

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While the size of the United States makes it hard for it to compete with the inter-city train access available in places like Japan and many European countries, Amtrak trains are a very popular transportation option in certain pockets of the country — so much so that the country’s national railway company is expanding its Northeast Corridor by more than one million seats.

Related: This is what it's like to take a 19-hour train from New York to Chicago

Running from Boston all the way south to Washington, D.C., the route is one of the most popular as it passes through the most densely populated part of the country and serves as a commuter train for those who need to go between East Coast cities such as New York and Philadelphia for business.

Veronika Bondarenko captured this photo of New York’s Moynihan Train Hall. 

Veronika Bondarenko

Amtrak launches new routes, promises travelers ‘additional travel options’

Earlier this month, Amtrak announced that it was adding four additional Northeastern routes to its schedule — two more routes between New York’s Penn Station and Union Station in Washington, D.C. on the weekend, a new early-morning weekday route between New York and Philadelphia’s William H. Gray III 30th Street Station and a weekend route between Philadelphia and Boston’s South Station.

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According to Amtrak, these additions will increase Northeast Corridor’s service by 20% on the weekdays and 10% on the weekends for a total of one million additional seats when counted by how many will ride the corridor over the year.

“More people are taking the train than ever before and we’re proud to offer our customers additional travel options when they ride with us on the Northeast Regional,” Amtrak Executive Vice President and Chief Commercial Officer Eliot Hamlisch said in a statement on the new routes. “The Northeast Regional gets you where you want to go comfortably, conveniently and sustainably as you breeze past traffic on I-95 for a more enjoyable travel experience.”

Here are some of the other Amtrak changes you can expect to see

Amtrak also said that, in the 2023 financial year, the Northeast Corridor had nearly 9.2 million riders — 8% more than it had pre-pandemic and a 29% increase from 2022. The higher demand, particularly during both off-peak hours and the time when many business travelers use to get to work, is pushing Amtrak to invest into this corridor in particular.

To reach more customers, Amtrak has also made several changes to both its routes and pricing system. In the fall of 2023, it introduced a type of new “Night Owl Fare” — if traveling during very late or very early hours, one can go between cities like New York and Philadelphia or Philadelphia and Washington. D.C. for $5 to $15.

As travel on the same routes during peak hours can reach as much as $300, this was a deliberate move to reach those who have the flexibility of time and might have otherwise preferred more affordable methods of transportation such as the bus. After seeing strong uptake, Amtrak added this type of fare to more Boston routes.

The largest distances, such as the ones between Boston and New York or New York and Washington, are available at the lowest rate for $20.

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The next pandemic? It’s already here for Earth’s wildlife

Bird flu is decimating species already threatened by climate change and habitat loss.

I am a conservation biologist who studies emerging infectious diseases. When people ask me what I think the next pandemic will be I often say that we are in the midst of one – it’s just afflicting a great many species more than ours.

I am referring to the highly pathogenic strain of avian influenza H5N1 (HPAI H5N1), otherwise known as bird flu, which has killed millions of birds and unknown numbers of mammals, particularly during the past three years.

This is the strain that emerged in domestic geese in China in 1997 and quickly jumped to humans in south-east Asia with a mortality rate of around 40-50%. My research group encountered the virus when it killed a mammal, an endangered Owston’s palm civet, in a captive breeding programme in Cuc Phuong National Park Vietnam in 2005.

How these animals caught bird flu was never confirmed. Their diet is mainly earthworms, so they had not been infected by eating diseased poultry like many captive tigers in the region.

This discovery prompted us to collate all confirmed reports of fatal infection with bird flu to assess just how broad a threat to wildlife this virus might pose.

This is how a newly discovered virus in Chinese poultry came to threaten so much of the world’s biodiversity.

H5N1 originated on a Chinese poultry farm in 1997. ChameleonsEye/Shutterstock

The first signs

Until December 2005, most confirmed infections had been found in a few zoos and rescue centres in Thailand and Cambodia. Our analysis in 2006 showed that nearly half (48%) of all the different groups of birds (known to taxonomists as “orders”) contained a species in which a fatal infection of bird flu had been reported. These 13 orders comprised 84% of all bird species.

We reasoned 20 years ago that the strains of H5N1 circulating were probably highly pathogenic to all bird orders. We also showed that the list of confirmed infected species included those that were globally threatened and that important habitats, such as Vietnam’s Mekong delta, lay close to reported poultry outbreaks.

Mammals known to be susceptible to bird flu during the early 2000s included primates, rodents, pigs and rabbits. Large carnivores such as Bengal tigers and clouded leopards were reported to have been killed, as well as domestic cats.

Our 2006 paper showed the ease with which this virus crossed species barriers and suggested it might one day produce a pandemic-scale threat to global biodiversity.

Unfortunately, our warnings were correct.

A roving sickness

Two decades on, bird flu is killing species from the high Arctic to mainland Antarctica.

In the past couple of years, bird flu has spread rapidly across Europe and infiltrated North and South America, killing millions of poultry and a variety of bird and mammal species. A recent paper found that 26 countries have reported at least 48 mammal species that have died from the virus since 2020, when the latest increase in reported infections started.

Not even the ocean is safe. Since 2020, 13 species of aquatic mammal have succumbed, including American sea lions, porpoises and dolphins, often dying in their thousands in South America. A wide range of scavenging and predatory mammals that live on land are now also confirmed to be susceptible, including mountain lions, lynx, brown, black and polar bears.

The UK alone has lost over 75% of its great skuas and seen a 25% decline in northern gannets. Recent declines in sandwich terns (35%) and common terns (42%) were also largely driven by the virus.

Scientists haven’t managed to completely sequence the virus in all affected species. Research and continuous surveillance could tell us how adaptable it ultimately becomes, and whether it can jump to even more species. We know it can already infect humans – one or more genetic mutations may make it more infectious.

At the crossroads

Between January 1 2003 and December 21 2023, 882 cases of human infection with the H5N1 virus were reported from 23 countries, of which 461 (52%) were fatal.

Of these fatal cases, more than half were in Vietnam, China, Cambodia and Laos. Poultry-to-human infections were first recorded in Cambodia in December 2003. Intermittent cases were reported until 2014, followed by a gap until 2023, yielding 41 deaths from 64 cases. The subtype of H5N1 virus responsible has been detected in poultry in Cambodia since 2014. In the early 2000s, the H5N1 virus circulating had a high human mortality rate, so it is worrying that we are now starting to see people dying after contact with poultry again.

It’s not just H5 subtypes of bird flu that concern humans. The H10N1 virus was originally isolated from wild birds in South Korea, but has also been reported in samples from China and Mongolia.

Recent research found that these particular virus subtypes may be able to jump to humans after they were found to be pathogenic in laboratory mice and ferrets. The first person who was confirmed to be infected with H10N5 died in China on January 27 2024, but this patient was also suffering from seasonal flu (H3N2). They had been exposed to live poultry which also tested positive for H10N5.

Species already threatened with extinction are among those which have died due to bird flu in the past three years. The first deaths from the virus in mainland Antarctica have just been confirmed in skuas, highlighting a looming threat to penguin colonies whose eggs and chicks skuas prey on. Humboldt penguins have already been killed by the virus in Chile.

A colony of king penguins.
Remote penguin colonies are already threatened by climate change. AndreAnita/Shutterstock

How can we stem this tsunami of H5N1 and other avian influenzas? Completely overhaul poultry production on a global scale. Make farms self-sufficient in rearing eggs and chicks instead of exporting them internationally. The trend towards megafarms containing over a million birds must be stopped in its tracks.

To prevent the worst outcomes for this virus, we must revisit its primary source: the incubator of intensive poultry farms.

Diana Bell does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.

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This is the biggest money mistake you’re making during travel

A retail expert talks of some common money mistakes travelers make on their trips.

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Travel is expensive. Despite the explosion of travel demand in the two years since the world opened up from the pandemic, survey after survey shows that financial reasons are the biggest factor keeping some from taking their desired trips.

Airfare, accommodation as well as food and entertainment during the trip have all outpaced inflation over the last four years.

Related: This is why we're still spending an insane amount of money on travel

But while there are multiple tricks and “travel hacks” for finding cheaper plane tickets and accommodation, the biggest financial mistake that leads to blown travel budgets is much smaller and more insidious.

A traveler watches a plane takeoff at an airport gate.

Jeshoots on Unsplash

This is what you should (and shouldn’t) spend your money on while abroad

“When it comes to traveling, it's hard to resist buying items so you can have a piece of that memory at home,” Kristen Gall, a retail expert who heads the financial planning section at points-back platform Rakuten, told Travel + Leisure in an interview. “However, it's important to remember that you don't need every souvenir that catches your eye.”

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According to Gall, souvenirs not only have a tendency to add up in price but also weight which can in turn require one to pay for extra weight or even another suitcase at the airport — over the last two months, airlines like Delta  (DAL) , American Airlines  (AAL)  and JetBlue Airways  (JBLU)  have all followed each other in increasing baggage prices to in some cases as much as $60 for a first bag and $100 for a second one.

While such extras may not seem like a lot compared to the thousands one might have spent on the hotel and ticket, they all have what is sometimes known as a “coffee” or “takeout effect” in which small expenses can lead one to overspend by a large amount.

‘Save up for one special thing rather than a bunch of trinkets…’

“When traveling abroad, I recommend only purchasing items that you can't get back at home, or that are small enough to not impact your luggage weight,” Gall said. “If you’re set on bringing home a souvenir, save up for one special thing, rather than wasting your money on a bunch of trinkets you may not think twice about once you return home.”

Along with the immediate costs, there is also the risk of purchasing things that go to waste when returning home from an international vacation. Alcohol is subject to airlines’ liquid rules while certain types of foods, particularly meat and other animal products, can be confiscated by customs. 

While one incident of losing an expensive bottle of liquor or cheese brought back from a country like France will often make travelers forever careful, those who travel internationally less frequently will often be unaware of specific rules and be forced to part with something they spent money on at the airport.

“It's important to keep in mind that you're going to have to travel back with everything you purchased,” Gall continued. “[…] Be careful when buying food or wine, as it may not make it through customs. Foods like chocolate are typically fine, but items like meat and produce are likely prohibited to come back into the country.

Related: Veteran fund manager picks favorite stocks for 2024

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