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I have been following the COVID-related travel restrictions and there is something that I do not understand: in some countries travellers can skip quarantine if they are vaccinated.

However, why instead of saying if you they are vaccinated or not, are not they checking if the person has antibodies against coronavirus?

People can get the antibodies by getting infected with the virus. The antibodies will probably decrease in time (no matter if they were produced because of the infection or due to the vaccine).

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    Note that some governments take into account for certain measures the fact that you previously had the virus and recovered. The details vary, but that usually involves a positive test followed by either a negative test or a doctor's note saying you have recovered. There are also time limits (minimum and maximum time since infection).
    – jcaron
    May 27 at 9:24
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    Coronaviruses are a whole family, not just Covid-19. Some of them have been quite common in the human population. Depending on the specificity of the test, exposure to one of the common coronaviruses might yield a positive antibody test, which would not reflect how Covid-19 would impact you.
    – Jon Custer
    May 27 at 16:51
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    I’m voting to close this question because it is not about travel but about medical/biological aspects of the virus. Maybe it is better fit for BiologySE
    – RedBaron
    May 28 at 4:39
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    One reason a person would have antibodies would be if they had an active CoViD infection, in which case you most certainly do not want them entering your country.
    – Vikki
    May 29 at 0:44
  • @RedBaron It's a mix of biology, public health, politics and trying to mindread government and airline decisions which in many cases do not take reasonable decisions. Probably not even the people that made the policy can answer the question.
    – jinawee
    May 29 at 22:51
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Commercially available antibody tests are not intended to be used to determine vaccination status, aren't tested to be effective for that purpose, and are known to sometimes produce negative results in vaccinated individuals who nonetheless have immunity. The test detects specific types of antibodies depending on its design, but that's not the same thing as a general measure of immunity to COVID-19. As the US CDC explains:

Antibody testing is not currently recommended to assess for immunity to COVID-19 following COVID-19 vaccination or to assess the need for vaccination in an unvaccinated person. Since vaccines induce antibodies to specific viral protein targets, post-vaccination serologic test results will be negative in persons without history of previous natural infection if the test used does not detect antibodies induced by the vaccine.

In other words, the antibodies your body makes in response to a vaccine may not be the same antibodies the test is looking for.

Or see Can antibody tests tell you if a COVID-19 vaccine worked?:

A vaccinated person is very likely to get a negative result from a serology test, even if the vaccine was successful and protective. That’s because different serology tests detect antibodies to different parts of the virus.

Some tests detect antibodies to the spike protein of the virus, which are produced in response to viral infection or the vaccine. Others detect antibodies to a different part of the virus called the nucleocapsid protein, which are produced in response to infection, but not by the current vaccines.

MD Anderson’s Blood Bank uses an antibody test designed to detect antibodies to the nucleocapsid protein, which means donors who have received the COVID-19 vaccine will likely receive a negative antibody test result.

As a result, a requirement for an antibody test would not serve the intended purpose, as some vaccinated travelers would have negative test results despite being well-protected, as the tests aren't designed to detect immunity from vaccination (though this depends on the exact combination of vaccine and serology test used).

Some countries do offer exemptions from their testing and/or quarantine requirements for travelers with a recent (usually within a few months) documented history of COVID infection and recovery, which does cover the case you mentioned where someone has a degree of natural immunity from a previous infection.

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    As of right now, how long the protection from vaccines will last is not really something we know and is an area of ongoing research, along with the protection different vaccines provide against different variants of the virus. It's sounding likely that there will be additional booster shots recommended in some cases, but there will probably be different recommendations for that depending on which vaccine you've already received and guidance from your national health authorities. May 27 at 6:19
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    It strikes me personally as unlikely that there will be widespread use of serology testing to determine when additional vaccines are required, even if new tests are developed, given how much of a logistical headache that would be. Most vaccines that require additional doses usually use a time-based schedule where revaccination is recommended after a certain number of years if the risk still remains. May 27 at 6:22
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    I would assume, that during studies to determine immunity longevity they would use such serology analysis. That should limit the logistical headache, since once the studies are complete, it's not necessary anymore.
    – dunni
    May 27 at 6:26
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    @JonathanReez The question was why antibody tests aren't being used "instead of" vaccination certificates. I wouldn't consider a negative test after vaccination to be a "false negative" since the test is doing what it's supposed to; it's just being used for a purpose it wasn't designed for. It sounds like you're asking a different question: "why don't countries accept antibody test results in addition to vaccination or proof of recovery?" I'd imagine that uncertainty, caution, and lack of scientific evidence about immunity and reinfection are reasons there, but I'm not sure. May 27 at 18:21
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    @dunni During studies for longevity, the serology tests used are far different than the ones available for bulk testing of patient samples. Glancing at a few recent articles regarding antibodies from vaccines, the tests include both titrations against much wider sets of targets and deeper sequence analysis that's deep enough to infer B cell lineages. Your typical chemiluminescent (commercially seen as the Abbott architect), ELISA, or lateral flow (rapid) test isn't going to be nearly as robust, in order to make it affordable and scalable.
    – nanofarad
    May 28 at 21:36
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The low number of travel exemptions for people who have recovered from COVID stems from the discussions around "immunity passports", which was one proposed solution to enabling travel prior to widespread availability of vaccines. The first counter-argument was that its unknown if infection can actually protect against subsequent reinfections:

There is currently no evidence that people who have recovered from COVID-19 and have antibodies are protected from a second infection.

This is now known to be categorically false, with natural infections being almost as good as a vaccine but its sort of ignored by most governments as they're trying to maximize the number of people getting a shot.

The other concern was that people would purposefully try to get infected so that they could unlock travel and other privileges:

Some experts say online immunity certificates are unethical as they would give certain privileges to people who’ve contracted Covid-19 and may even encourage people to catch the disease in order to obtain immune status.


At this time governments will at most grant some travel privileges to people who have tested positive for COVID in the past 6 months, i.e. Germany has the following policy:

How do you prove you’re recovered from Covid-19?

People who have recovered from coronavirus and were infected no more than six months ago also face less restrictions. The test must have been a PCR test (or similar), checked in a lab and taken at least 28 days ago, and be no older than six months. An antibody test is not sufficient because antibodies can decrease over time.

The last paragraph makes absolutely no sense as the presence of false negatives doesn't mean positive results couldn't be used to determine who's immune to COVID. Most likely this rule was put into place to justify the 6 months limit. With antibody tests people could potentially get around the need to get vaccinated for as long as their antibodies would last. Without them, they can only do this for 6 months after getting infected, which excludes everyone who got infected in the first COVID wave.

It can be frustrating when government policies don't follow logic and reason but unfortunately there's nothing we can do about it. Some countries go even farther - i.e. Canada doesn't give vaccinated people any travel benefits at all, not even to their own citizens with a Canadian vaccination certificate - let alone to people who have recovered from the disease.

NB: I am personally fully vaccinated and got my vaccine on the very first day of availability to my age group. This post is in no way intended to disparage the vaccination campaign.

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    "...nothing we can do about it." Well... there's always revolt, either at the ballot box or otherwise. May 27 at 13:37
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    With antibody tests people could potentially get around the need to get vaccinated for as long as their antibodies would last. — This is the very point I still do not get: if the goal is that people should not get infected, and if antibodies do provide some protection against the virus, why would someone be forced to be vaccinated if they already have protection against the virus (antibodies) to skip quarantine / enter the country? May 27 at 15:04
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    @IonicăBizău because the governments want to maximize just one metric - "% of people vaccinated". Allowing people with antibodies to count as immune would complicate the calculus as you don't have an easy way to get your entire population get an antibody test. Tourism policies are just an extension of the domestic policy - if "get a shot" is the only approved way to become immune then logically speaking it would also be the only approved way for tourists.
    – JonathanReez
    May 27 at 16:50
  • Total plot to persuade people to vaccinate :/ Second goal for the really hard nuts - pay as much as possible for PCR tests. I actually like Canada's policy not to give more rights so that only people that want to vaccinate will do without the pressure. May 27 at 19:18
  • @akostadinov I wouldn’t say “plot”. It’s just incompetence and unwillingness to reason from first principles rather than through bureaucratic rulemaking.
    – JonathanReez
    May 27 at 19:53
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An important point I think worth mentioning (and is related to the idea of decreasing antibodies over time mentioned in the other answers) is that the goal of vaccines isn't necessarily just to create antibodies; it's to trigger long-lasting (hopefully) changes in the immune system (e.g., the formation of Memory B cells). These changes will then allow the body to respond more rapidly to future infections by doing things like ramping up antibody production faster than it would otherwise. So even if antibodies fade over time, we still have protection for some time after that because of these other underlying changes. It's why some vaccines can produce life-long protection (for things like measles and chickenpox).

The problem from a testing point of view (for things like travel) is that antibodies are relatively easy to test for compared to these other things (with the caveats and issues mentioned in other answers applying). Some of the changes may be detectable in the blood, but to really know a persons true immunity would require digging into other parts of our overall immune system, like the lymphatic system.

Some reading topics for those interested in more details:

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    Note that the chickenpox vaccine for kids is not life-long protection - a shingles vaccine later in life is still recommended.
    – Jon Custer
    May 27 at 16:20
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Something else to keep in mind is practical considerations related to antibody tests. Note that I'm not a doctor and as such might be wrong about certain things.

Antibody tests generally require bloodwork to be taken. Bloodwork means drawing blood from the patient. This brings many considerations compared to vaccinations and PCR tests related to how easily the test can be administered, who can administer it, how long it takes to administer it, where it can be administered and who is eligible to undergo these tests. There are also practical considerations related to the transport of the specimens and the test itself.

I'm not going to pretend like I know enough about these tests to know what these considerations are, but from the best of my understanding, people generally are more willing and able to get their sinuses/throat swabbed than they are to get blood drawn, and PCR tests generally are easier to scale up than bloodwork is. This may be part of the reason why countries may prefer PCR tests and vaccination reports over antibody tests.

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    PCR tests require a complex process and a machine, their avalaibility was a huge problem one year ago and they still cost more than blood tests, which are also quicker.
    – Relaxed
    May 28 at 13:03
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Logistics & Economy

Medical tests require:

  • Qualified crews.
  • Supplies
  • Time

Checking a document require a few seconds time of an entry level clerk.

Medical tests are way more expensive and disruptive than checking documents, and they escalate poorly.

I happen to be closely involved in the procedures for handling passengers at a local airport. Although they were advised to come with a PCR test results, those who did not have one would be subjected to an antibodies test.

And even with only a fraction of the passage affected, and with travel severely curtailed, it has already been a difficult task. You have to receive all the passengers, sort out who needs the tests, administer the tests, wait for the results, communicate them to the waiting passengers (and beware of privacy issues), getting sure that no passengers leave the testing area before getting the results...

If you want to allow travel on a significant basis, you just cannot rely on performing medical tests on arrival. It simply does not work. So you settle for the best next: assume that vaccinated people won't get infected, and let them pass freely.

Yes we do know that some vaccinated people may be infected and contagious, but even the risk of those who can be reinfected depends of the infection rate at their origin. So with low rates the governments are willing to bet on easing the restrictions on travel in order to estimulate the economy and appease the public. There is certainly an element of risk (as there has been any time restrictions have been eased) but they seem to expect to contain the situation at least until vaccination is high enough.

It is not that different from the current situation: even PCRs leave some room for not detecting infected people, yet may EU countries allow travel without quarantine with just a PCR. There is a need to balance the costs and risks of each safety measure.

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  • This explains why it couldn’t be done on arrival. But why couldn’t it be done pre departure by allowing passengers with an antibody certificate from a trusted lab to enter without restrictions? If a country allows this for vaccinated people they should also allow it for recovered people.
    – JonathanReez
    May 30 at 18:51
  • The European Digital Green Certificate (or whatever it is called at this time) will register negative results of analysis (PCR and antibodies). PCRs will be valid for 72 hours, antibodies for 24. Each issuing authority (in Spain case, regional health administration) can allow private laboratories to report results and certificates will be issued. But either at origin or destination there is not enough capacity to restore travel, and issuing a vaccination certificate is easy and cheap (I am into that, too).
    – SJuan76
    May 30 at 20:11
  • @JonathanReez Nitpick: I do know about the details of the generation of the certificate, but I do not know if all of the data collected will be enough to allow entry. For example, certificates will be issued for people with partial vaccination, but I do not know if all countries would allow those to cross the border or even if there will be a common policy or each country will chose what kind of status allows for travel (e.g. perhaps Spain allow tourist with only one vaccine dose but Germany will require full vaccination).
    – SJuan76
    May 30 at 20:16
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When you acquire an infection, antibodies will show up. You are infectious at this stage.

Remember the children chicken pox parties of yesterday-year that were attempts at conveying natural herd immunity? That debacle lead to an increase of Shingles (an adult version of chicken pox) which can be very debilitating and even deadly. Virus can remain dormant.

Also, the Covid19 vaccines seem to convey some level of anti-spreading properties. That is, you are less likely to be a carrier and to spread the actual Covid19 infection. That is the rationale behind the CDC saying that fully vaccinated persons can go around maskless around other vaccinated persons.

As others have pointed out, the antibody tests are notoriously unreliable. I recall in the real news that a lady in the US was tested negative 2 times using the antibody tests but later was hospitalized for Covid19. During the the second and third tests, she was even exhibiting sever symptoms. The final 3rd test, a PCR test showed positive for Covid19.

With the super spreader UK117 variant floating around, the current vaccines may not convey sufficient immunity. I would continue to wear a mask when I am around a crowd. Virus do not recognize sovereignty.

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